diff --git a/include/boost/json/detail/handler.hpp b/include/boost/json/detail/handler.hpp
index 6880d723d..2cf2f58d4 100644
--- a/include/boost/json/detail/handler.hpp
+++ b/include/boost/json/detail/handler.hpp
@@ -35,6 +35,7 @@ struct handler
         max_string_size = string::max_size();
 
     value_stack st;
+    bool ignore_duplicate_keys = true;
 
     template<class... Args>
     explicit
diff --git a/include/boost/json/detail/impl/handler.ipp b/include/boost/json/detail/impl/handler.ipp
index 20b40efa3..e7426524d 100644
--- a/include/boost/json/detail/impl/handler.ipp
+++ b/include/boost/json/detail/impl/handler.ipp
@@ -51,10 +51,10 @@ bool
 handler::
 on_object_end(
     std::size_t n,
-    error_code&)
+    error_code& ec)
 {
-    st.push_object(n);
-    return true;
+    ec = st.push_object(n, ignore_duplicate_keys);
+    return !ec.failed();
 }
 
 bool
@@ -85,7 +85,7 @@ on_key_part(
     st.push_chars(s);
     return true;
 }
-        
+
 bool
 handler::
 on_key(
@@ -96,12 +96,12 @@ on_key(
     st.push_key(s);
     return true;
 }
-        
+
 bool
 handler::
 on_string_part(
     string_view s,
-    std::size_t, 
+    std::size_t,
     error_code&)
 {
     st.push_chars(s);
@@ -112,7 +112,7 @@ bool
 handler::
 on_string(
     string_view s,
-    std::size_t, 
+    std::size_t,
     error_code&)
 {
     st.push_string(s);
@@ -138,7 +138,7 @@ on_int64(
     st.push_int64(i);
     return true;
 }
-        
+
 bool
 handler::
 on_uint64(
@@ -160,7 +160,7 @@ on_double(
     st.push_double(d);
     return true;
 }
-        
+
 bool
 handler::
 on_bool(
@@ -187,7 +187,7 @@ on_comment_part(
 {
     return true;
 }
-        
+
 bool
 handler::
 on_comment(
diff --git a/include/boost/json/detail/object.hpp b/include/boost/json/detail/object.hpp
index 73c1956a5..0044cceec 100644
--- a/include/boost/json/detail/object.hpp
+++ b/include/boost/json/detail/object.hpp
@@ -28,30 +28,29 @@ class unchecked_object
     // first one is a string key,
     // second one is the value.
     value* data_;
-    std::size_t size_;
+    value* end_;
     storage_ptr const& sp_;
+    bool ignore_duplicates_;
 
 public:
     inline
     ~unchecked_object();
 
+    inline
     unchecked_object(
         value* data,
         std::size_t size, // # of kv-pairs
-        storage_ptr const& sp) noexcept
-        : data_(data)
-        , size_(size)
-        , sp_(sp)
-    {
-    }
+        storage_ptr const& sp,
+        bool ignore_duplicates) noexcept;
 
     unchecked_object(
         unchecked_object&& other) noexcept
         : data_(other.data_)
-        , size_(other.size_)
+        , end_(other.end_)
         , sp_(other.sp_)
+        , ignore_duplicates_(other.ignore_duplicates_)
     {
-        other.data_ = nullptr;
+        other.data_ = other.end_ = nullptr;
     }
 
     storage_ptr const&
@@ -60,19 +59,25 @@ class unchecked_object
         return sp_;
     }
 
+    inline
     std::size_t
-    size() const noexcept
+    size() const noexcept;
+
+    bool
+    ignore_duplicate_keys() const noexcept
     {
-        return size_;
+        return ignore_duplicates_;
     }
 
     value*
-    release() noexcept
+    front() noexcept
     {
-        auto const data = data_;
-        data_ = nullptr;
-        return data;
+        return data_;
     }
+
+    inline
+    void
+    pop_front() noexcept;
 };
 
 template<class CharRange>
@@ -88,6 +93,9 @@ find_in_object<string_view>(
     object const&,
     string_view key) noexcept;
 
+template< bool SmallTable, bool IgnoreDuplicates >
+void init_from_unchecked( object& obj, unchecked_object& uo );
+
 } // detail
 BOOST_JSON_NS_END
 
diff --git a/include/boost/json/error.hpp b/include/boost/json/error.hpp
index 6b19ee039..863ca8e5c 100644
--- a/include/boost/json/error.hpp
+++ b/include/boost/json/error.hpp
@@ -66,6 +66,9 @@ enum class error
     /// error occured when trying to read input
     input_error,
 
+    /// duplicate object key
+    duplicate_key,
+
     //
     // generic errors
     //
diff --git a/include/boost/json/impl/error.ipp b/include/boost/json/impl/error.ipp
index 09d57fcfb..ec508ac96 100644
--- a/include/boost/json/impl/error.ipp
+++ b/include/boost/json/impl/error.ipp
@@ -45,6 +45,7 @@ case error::array_too_large: return "array too large";
 case error::key_too_large: return "key too large";
 case error::string_too_large: return "string too large";
 case error::input_error: return "input error";
+case error::duplicate_key: return "duplicate key";
 
 case error::exception: return "got exception";
 case error::test_failure: return "test failure";
@@ -93,6 +94,7 @@ case error::array_too_large:
 case error::key_too_large:
 case error::string_too_large:
 case error::input_error:
+case error::duplicate_key:
     return condition::parse_error;
 
 case error::missing_slash:
diff --git a/include/boost/json/impl/object.hpp b/include/boost/json/impl/object.hpp
index 628f4479b..82d67f430 100644
--- a/include/boost/json/impl/object.hpp
+++ b/include/boost/json/impl/object.hpp
@@ -518,19 +518,43 @@ namespace detail {
 unchecked_object::
 ~unchecked_object()
 {
-    if(! data_)
-        return;
     if(sp_.is_not_shared_and_deallocate_is_trivial())
         return;
-    value* p = data_;
-    while(size_--)
+
+    for( value* p = data_; p != end_; p += 2 )
     {
         p[0].~value();
         p[1].~value();
-        p += 2;
     }
 }
 
+unchecked_object::
+unchecked_object(
+    value* data,
+    std::size_t size,
+    storage_ptr const& sp,
+    bool ignore_duplicates) noexcept
+    : data_(data)
+    , end_(data + 2 * size)
+    , sp_(sp)
+    , ignore_duplicates_(ignore_duplicates)
+{
+}
+
+std::size_t
+unchecked_object::
+size() const noexcept
+{
+    return std::size_t(end_ - data_) / 2;
+}
+
+void
+unchecked_object::
+pop_front() noexcept
+{
+    data_ += 2;
+}
+
 } // detail
 
 BOOST_JSON_NS_END
diff --git a/include/boost/json/impl/object.ipp b/include/boost/json/impl/object.ipp
index 28f732db1..bfdbea4b0 100644
--- a/include/boost/json/impl/object.ipp
+++ b/include/boost/json/impl/object.ipp
@@ -70,6 +70,75 @@ find_in_object<string_view>(
     object const& obj,
     string_view key) noexcept;
 
+template< bool SmallTable, bool IgnoreDuplicates >
+void init_from_unchecked( object& obj, unchecked_object& uo )
+{
+    // insert all elements, keeping
+    // the last of any duplicate keys, unless IgnoreDuplicates is false.
+    auto const begin = obj.begin();
+    auto dest = begin;
+    for( ; uo.size(); uo.pop_front() )
+    {
+        auto src = uo.front();
+        access::construct_key_value_pair(
+            dest, pilfer(src[0]), pilfer(src[1]));
+
+        string_view const key = dest->key();
+        key_value_pair* duplicate = nullptr;
+        BOOST_IF_CONSTEXPR ( SmallTable )
+            duplicate = find_in_object( obj, key ).first;
+        else
+        {
+            auto& head = obj.t_->bucket( key );
+            auto i = head;
+            while( true )
+            {
+                if( i == object::null_index_ )
+                {
+                    // end of bucket
+                    access::next(*dest) = head;
+                    head = static_cast<object::index_t>( dest - begin );
+                    break;
+                }
+                auto& v = begin[i];
+                if( v.key() != key )
+                {
+                    i = access::next(v);
+                    continue;
+                }
+
+                // handle duplicate
+                access::next(*dest) = access::next(v);
+                duplicate = &v;
+                break;
+            }
+        }
+
+        if( !duplicate )
+        {
+            ++dest;
+            ++obj.t_->size;
+            continue;
+        }
+
+        // handle duplicate
+        BOOST_IF_CONSTEXPR ( IgnoreDuplicates )
+        {
+            // don't bother to check if
+            // storage deallocate is trivial
+            duplicate->~key_value_pair();
+            // trivial relocate
+            std::memcpy(
+                static_cast<void*>(duplicate), dest, sizeof(key_value_pair) );
+        }
+        else
+        {
+            dest->~key_value_pair();
+            return;
+        }
+    }
+}
+
 } // namespace detail
 
 //----------------------------------------------------------
@@ -197,7 +266,7 @@ destroy() noexcept
 //----------------------------------------------------------
 
 object::
-object(detail::unchecked_object&& uo)
+object(detail::unchecked_object& uo)
     : sp_(uo.storage())
 {
     if(uo.size() == 0)
@@ -210,80 +279,18 @@ object(detail::unchecked_object&& uo)
         uo.size() <= max_size());
     t_ = table::allocate(
         uo.size(), 0, sp_);
+    t_->size = 0;
 
-    // insert all elements, keeping
-    // the last of any duplicate keys.
-    auto dest = begin();
-    auto src = uo.release();
-    auto const end = src + 2 * uo.size();
     if(t_->is_small())
-    {
-        t_->size = 0;
-        while(src != end)
-        {
-            access::construct_key_value_pair(
-                dest, pilfer(src[0]), pilfer(src[1]));
-            src += 2;
-            auto result = detail::find_in_object(*this, dest->key());
-            if(! result.first)
-            {
-                ++dest;
-                ++t_->size;
-                continue;
-            }
-            // handle duplicate
-            auto& v = *result.first;
-            // don't bother to check if
-            // storage deallocate is trivial
-            v.~key_value_pair();
-            // trivial relocate
-            std::memcpy(
-                static_cast<void*>(&v),
-                    dest, sizeof(v));
-        }
-        return;
-    }
-    while(src != end)
-    {
-        access::construct_key_value_pair(
-            dest, pilfer(src[0]), pilfer(src[1]));
-        src += 2;
-        auto& head = t_->bucket(dest->key());
-        auto i = head;
-        for(;;)
-        {
-            if(i == null_index_)
-            {
-                // end of bucket
-                access::next(
-                    *dest) = head;
-                head = static_cast<index_t>(
-                    dest - begin());
-                ++dest;
-                break;
-            }
-            auto& v = (*t_)[i];
-            if(v.key() != dest->key())
-            {
-                i = access::next(v);
-                continue;
-            }
-
-            // handle duplicate
-            access::next(*dest) =
-                access::next(v);
-            // don't bother to check if
-            // storage deallocate is trivial
-            v.~key_value_pair();
-            // trivial relocate
-            std::memcpy(
-                static_cast<void*>(&v),
-                    dest, sizeof(v));
-            break;
-        }
-    }
-    t_->size = static_cast<
-        index_t>(dest - begin());
+        if( uo.ignore_duplicate_keys() )
+            detail::init_from_unchecked< true, true >( *this, uo );
+        else
+            detail::init_from_unchecked< true, false >( *this, uo );
+    else
+        if( uo.ignore_duplicate_keys() )
+            detail::init_from_unchecked< false, true >( *this, uo );
+        else
+            detail::init_from_unchecked< false, false >( *this, uo );
 }
 
 object::
diff --git a/include/boost/json/impl/parser.ipp b/include/boost/json/impl/parser.ipp
index 6c17bd42c..399b5f8cd 100644
--- a/include/boost/json/impl/parser.ipp
+++ b/include/boost/json/impl/parser.ipp
@@ -32,20 +32,19 @@ parser(
         size)
 {
     reset();
+    p_.handler().ignore_duplicate_keys = opt.ignore_duplicate_keys;
 }
 
 parser::
 parser(
     storage_ptr sp,
     parse_options const& opt) noexcept
-    : p_(
-        opt,
+    : parser(
         std::move(sp),
-        nullptr,
+        opt,
+        static_cast<unsigned char*>(nullptr),
         0)
-{
-    reset();
-}
+{ }
 
 void
 parser::
diff --git a/include/boost/json/impl/stream_parser.ipp b/include/boost/json/impl/stream_parser.ipp
index b5b798ac2..37fe6c0ea 100644
--- a/include/boost/json/impl/stream_parser.ipp
+++ b/include/boost/json/impl/stream_parser.ipp
@@ -32,20 +32,19 @@ stream_parser(
         size)
 {
     reset();
+    p_.handler().ignore_duplicate_keys = opt.ignore_duplicate_keys;
 }
 
 stream_parser::
 stream_parser(
     storage_ptr sp,
     parse_options const& opt) noexcept
-    : p_(
-        opt,
+    : stream_parser(
         std::move(sp),
-        nullptr,
+        opt,
+        static_cast<unsigned char*>(nullptr),
         0)
-{
-    reset();
-}
+{ }
 
 void
 stream_parser::
diff --git a/include/boost/json/impl/value_stack.ipp b/include/boost/json/impl/value_stack.ipp
index 1e23b0042..11ad65cf3 100644
--- a/include/boost/json/impl/value_stack.ipp
+++ b/include/boost/json/impl/value_stack.ipp
@@ -289,7 +289,7 @@ exchange(Unchecked&& u)
     // which belongs to `u`.
     detail::access::
         construct_value(
-            &jv.v, std::move(u));
+            &jv.v, static_cast<Unchecked&&>(u));
     std::memcpy(
         reinterpret_cast<
             char*>(top_),
@@ -364,16 +364,25 @@ push_array(std::size_t n)
     st_.exchange(std::move(ua));
 }
 
-void
+error_code
 value_stack::
-push_object(std::size_t n)
+push_object(std::size_t n, bool ignore_duplicates)
 {
     // we already have room if n > 0
     if(BOOST_JSON_UNLIKELY(n == 0))
         st_.maybe_grow();
+
     detail::unchecked_object uo(
-        st_.release(n * 2), n, sp_);
-    st_.exchange(std::move(uo));
+        st_.release(n * 2), n, sp_, ignore_duplicates);
+    st_.exchange(uo);
+
+    error_code ec;
+    // constructed object should have consumed all of uo's data
+    if( uo.size() )
+    {
+        BOOST_JSON_FAIL( ec, error::duplicate_key );
+    }
+    return ec;
 }
 
 void
diff --git a/include/boost/json/object.hpp b/include/boost/json/object.hpp
index a4ef9351f..516fc2558 100644
--- a/include/boost/json/object.hpp
+++ b/include/boost/json/object.hpp
@@ -90,7 +90,7 @@ class object
 
     BOOST_JSON_DECL
     explicit
-    object(detail::unchecked_object&& uo);
+    object(detail::unchecked_object& uo);
 
 public:
     /** The type of _Allocator_ returned by @ref get_allocator
@@ -1646,6 +1646,11 @@ class object
     reindex_relocate(
         key_value_pair* src,
         key_value_pair* dst) noexcept;
+
+    template< bool SmallTable, bool IgnoreDuplicates >
+    friend
+    void
+    detail::init_from_unchecked( object& obj, detail::unchecked_object& uo );
 };
 
 BOOST_JSON_NS_END
diff --git a/include/boost/json/parse_options.hpp b/include/boost/json/parse_options.hpp
index 2b3fee182..206a29eb5 100644
--- a/include/boost/json/parse_options.hpp
+++ b/include/boost/json/parse_options.hpp
@@ -76,6 +76,20 @@ struct parse_options
             @ref stream_parser.
     */
     bool allow_invalid_utf8 = false;
+
+
+    /** Unique keys restriction setting
+
+        Forbid duplicate keys to appear in objects.
+
+        @note Since @ref basic_parser doesn't store parsed elements directly,
+        this option has to be taken account by implementers of handlers.
+
+        @see
+            @ref basic_parser,
+            @ref stream_parser.
+    */
+    bool ignore_duplicate_keys = true;
 };
 
 BOOST_JSON_NS_END
diff --git a/include/boost/json/value.hpp b/include/boost/json/value.hpp
index c5527d4d6..f4efb7b73 100644
--- a/include/boost/json/value.hpp
+++ b/include/boost/json/value.hpp
@@ -81,8 +81,8 @@ class value
 
     explicit
     value(
-        detail::unchecked_object&& uo)
-        : obj_(std::move(uo))
+        detail::unchecked_object& uo)
+        : obj_(uo)
     {
     }
 
diff --git a/include/boost/json/value_stack.hpp b/include/boost/json/value_stack.hpp
index 885f8d2b3..67061aa51 100644
--- a/include/boost/json/value_stack.hpp
+++ b/include/boost/json/value_stack.hpp
@@ -13,6 +13,7 @@
 #include <boost/json/detail/config.hpp>
 #include <boost/json/error.hpp>
 #include <boost/json/storage_ptr.hpp>
+#include <boost/json/system_error.hpp>
 #include <boost/json/value.hpp>
 #include <stddef.h>
 
@@ -355,8 +356,8 @@ class value_stack
         top of the stack to form the array.
     */
     BOOST_JSON_DECL
-    void
-    push_object(std::size_t n);
+    error_code
+    push_object(std::size_t n, bool ignore_duplicates = true);
 
     /** Push part of a key or string onto the stack.
 
diff --git a/test/error.cpp b/test/error.cpp
index ad57dc3d0..d2f37b24b 100644
--- a/test/error.cpp
+++ b/test/error.cpp
@@ -61,6 +61,7 @@ class error_test
         check(condition::parse_error, error::key_too_large);
         check(condition::parse_error, error::string_too_large);
         check(condition::parse_error, error::input_error);
+        check(condition::parse_error, error::duplicate_key);
 
         check(condition::pointer_parse_error, error::missing_slash);
         check(condition::pointer_parse_error, error::invalid_escape);
diff --git a/test/parse.cpp b/test/parse.cpp
index 41405f49b..48cd3ce6a 100644
--- a/test/parse.cpp
+++ b/test/parse.cpp
@@ -203,6 +203,32 @@ class parse_test
         BOOST_TEST_THROWS( parse(ss), system_error );
     }
 
+    void
+    testDuplicates()
+    {
+        value jv = parse( R"( {"a": 1, "a": 2} )" );
+        BOOST_TEST( jv.as_object().size() == 1 );
+
+        parse_options opt;
+        opt.ignore_duplicate_keys = false;
+
+        error_code ec;
+        jv = parse( R"( {"a": 1, "a": 2} )", ec, {}, opt );
+        BOOST_TEST( ec == error::duplicate_key );
+
+        ec.clear();
+        std::string s;
+        s = "{\"999\":null";
+        for(int i = 1; i < 1000; ++i)
+            s +=
+                ",\"" +
+                std::to_string(i) +
+                "\":null";
+        s.append("}");
+        jv = parse( s, ec, {}, opt );
+        BOOST_TEST( ec == error::duplicate_key );
+    }
+
     void
     run()
     {
@@ -210,6 +236,7 @@ class parse_test
         testMemoryUsage();
         testIssue726();
         testIstream();
+        testDuplicates();
     }
 };