diff --git a/include/boost/json/detail/impl/handler.ipp b/include/boost/json/detail/impl/handler.ipp
index dba0841bb..e7426524d 100644
--- a/include/boost/json/detail/impl/handler.ipp
+++ b/include/boost/json/detail/impl/handler.ipp
@@ -53,13 +53,8 @@ on_object_end(
     std::size_t n,
     error_code& ec)
 {
-    if( !ignore_duplicate_keys )
-        ec = st.check_duplicates(n);
-    if( ec.failed() )
-        return false;
-
-    st.push_object(n);
-    return true;
+    ec = st.push_object(n, ignore_duplicate_keys);
+    return !ec.failed();
 }
 
 bool
@@ -90,7 +85,7 @@ on_key_part(
     st.push_chars(s);
     return true;
 }
-        
+
 bool
 handler::
 on_key(
@@ -101,12 +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);
@@ -117,7 +112,7 @@ bool
 handler::
 on_string(
     string_view s,
-    std::size_t, 
+    std::size_t,
     error_code&)
 {
     st.push_string(s);
@@ -143,7 +138,7 @@ on_int64(
     st.push_int64(i);
     return true;
 }
-        
+
 bool
 handler::
 on_uint64(
@@ -165,7 +160,7 @@ on_double(
     st.push_double(d);
     return true;
 }
-        
+
 bool
 handler::
 on_bool(
@@ -192,7 +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..ccf96f65b 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>
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..e683dc973 100644
--- a/include/boost/json/impl/object.ipp
+++ b/include/boost/json/impl/object.ipp
@@ -197,7 +197,7 @@ destroy() noexcept
 //----------------------------------------------------------
 
 object::
-object(detail::unchecked_object&& uo)
+object(detail::unchecked_object& uo)
     : sp_(uo.storage())
 {
     if(uo.size() == 0)
@@ -210,20 +210,18 @@ 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.
+    // the last of any duplicate keys, unless uo.ignore_duplicates is false.
     auto dest = begin();
-    auto src = uo.release();
-    auto const end = src + 2 * uo.size();
     if(t_->is_small())
     {
-        t_->size = 0;
-        while(src != end)
+        for( ; uo.size(); uo.pop_front() )
         {
+            auto src = uo.front();
             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)
             {
@@ -232,6 +230,11 @@ object(detail::unchecked_object&& uo)
                 continue;
             }
             // handle duplicate
+            if( !uo.ignore_duplicate_keys() )
+            {
+                dest->~key_value_pair();
+                return;
+            }
             auto& v = *result.first;
             // don't bother to check if
             // storage deallocate is trivial
@@ -243,11 +246,11 @@ object(detail::unchecked_object&& uo)
         }
         return;
     }
-    while(src != end)
+    for( ; uo.size() ; uo.pop_front() )
     {
+        auto src = uo.front();
         access::construct_key_value_pair(
             dest, pilfer(src[0]), pilfer(src[1]));
-        src += 2;
         auto& head = t_->bucket(dest->key());
         auto i = head;
         for(;;)
@@ -260,6 +263,7 @@ object(detail::unchecked_object&& uo)
                 head = static_cast<index_t>(
                     dest - begin());
                 ++dest;
+                ++t_->size;
                 break;
             }
             auto& v = (*t_)[i];
@@ -270,6 +274,11 @@ object(detail::unchecked_object&& uo)
             }
 
             // handle duplicate
+            if( !uo.ignore_duplicate_keys() )
+            {
+                dest->~key_value_pair();
+                return;
+            }
             access::next(*dest) =
                 access::next(v);
             // don't bother to check if
@@ -282,8 +291,6 @@ object(detail::unchecked_object&& uo)
             break;
         }
     }
-    t_->size = static_cast<
-        index_t>(dest - begin());
 }
 
 object::
diff --git a/include/boost/json/impl/value_stack.ipp b/include/boost/json/impl/value_stack.ipp
index 71c6e6f3d..11ad65cf3 100644
--- a/include/boost/json/impl/value_stack.ipp
+++ b/include/boost/json/impl/value_stack.ipp
@@ -82,37 +82,6 @@ has_chars()
     return chars_ != 0;
 }
 
-error_code
-value_stack::
-stack::
-check_duplicates(std::size_t n)
-{
-    error_code ec;
-
-    for( value* first = top_ - 2 * n; first != top_; first += 2 )
-    {
-        BOOST_ASSERT( first->is_string() );
-        value* other = first + 2;
-        while( true )
-        {
-            BOOST_ASSERT( other->is_string() );
-            if( first->get_string() == other->get_string() )
-            {
-                BOOST_JSON_FAIL( ec, error::duplicate_key );
-                goto before_return;
-            }
-
-            if( other == top_ )
-                break;
-
-            other += 2;
-        }
-    }
-
-before_return:
-    return ec;
-}
-
 //--------------------------------------
 
 // destroy the values but
@@ -320,7 +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_),
@@ -395,16 +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
@@ -498,13 +476,6 @@ push_null()
     st_.push(nullptr, sp_);
 }
 
-error_code
-value_stack::
-check_duplicates(std::size_t n)
-{
-    return st_.check_duplicates(n);
-}
-
 BOOST_JSON_NS_END
 
 #endif
diff --git a/include/boost/json/object.hpp b/include/boost/json/object.hpp
index a4ef9351f..1c59f48e9 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
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 bd3b447e5..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>
 
@@ -140,7 +141,6 @@ class value_stack
         inline void run_dtors(bool b) noexcept;
         inline std::size_t size() const noexcept;
         inline bool has_chars();
-        inline error_code check_duplicates(std::size_t n);
 
         inline void clear() noexcept;
         inline void maybe_grow();
@@ -356,8 +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.
 
@@ -502,10 +502,6 @@ class value_stack
     BOOST_JSON_DECL
     void
     push_null();
-
-    BOOST_JSON_DECL
-    error_code
-    check_duplicates(std::size_t n);
 };
 
 BOOST_JSON_NS_END
diff --git a/test/parse.cpp b/test/parse.cpp
index 1a7ef19b2..48cd3ce6a 100644
--- a/test/parse.cpp
+++ b/test/parse.cpp
@@ -210,11 +210,23 @@ class parse_test
         BOOST_TEST( jv.as_object().size() == 1 );
 
         parse_options opt;
+        opt.ignore_duplicate_keys = false;
 
         error_code ec;
-        opt.ignore_duplicate_keys = false;
         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