diff --git a/mypyc/irbuild/mapper.py b/mypyc/irbuild/mapper.py
index 4364b2b6c511..a108766644ce 100644
--- a/mypyc/irbuild/mapper.py
+++ b/mypyc/irbuild/mapper.py
@@ -116,7 +116,18 @@ def type_to_rtype(self, typ: Type | None) -> RType:
         elif isinstance(typ, NoneTyp):
             return none_rprimitive
         elif isinstance(typ, UnionType):
-            return RUnion([self.type_to_rtype(item) for item in typ.items])
+            # Remove redundant items using set + list to preserve item order
+            seen = set()
+            items = []
+            for item in typ.items:
+                rtype = self.type_to_rtype(item)
+                if rtype not in seen:
+                    items.append(rtype)
+                    seen.add(rtype)
+            if len(items) > 1:
+                return RUnion(items)
+            else:
+                return items[0]
         elif isinstance(typ, AnyType):
             return object_rprimitive
         elif isinstance(typ, TypeType):
diff --git a/mypyc/test-data/irbuild-lists.test b/mypyc/test-data/irbuild-lists.test
index 47f7ada709e3..b82217465fef 100644
--- a/mypyc/test-data/irbuild-lists.test
+++ b/mypyc/test-data/irbuild-lists.test
@@ -428,3 +428,40 @@ L4:
 L5:
     res = r8
     return 1
+
+[case testSimplifyListUnion]
+from typing import List, Union
+
+def f(a: Union[List[str], List[bytes], int]) -> int:
+    if isinstance(a, list):
+        return len(a)
+    return a
+[out]
+def f(a):
+    a :: union[list, int]
+    r0 :: object
+    r1 :: int32
+    r2 :: bit
+    r3 :: bool
+    r4 :: list
+    r5 :: ptr
+    r6 :: native_int
+    r7 :: short_int
+    r8 :: int
+L0:
+    r0 = load_address PyList_Type
+    r1 = PyObject_IsInstance(a, r0)
+    r2 = r1 >= 0 :: signed
+    r3 = truncate r1: int32 to builtins.bool
+    if r3 goto L1 else goto L2 :: bool
+L1:
+    r4 = borrow cast(list, a)
+    r5 = get_element_ptr r4 ob_size :: PyVarObject
+    r6 = load_mem r5 :: native_int*
+    keep_alive r4
+    r7 = r6 << 1
+    keep_alive a
+    return r7
+L2:
+    r8 = unbox(int, a)
+    return r8
diff --git a/mypyc/test-data/irbuild-optional.test b/mypyc/test-data/irbuild-optional.test
index 4b1d3d1ffec2..e98cf1b19e2e 100644
--- a/mypyc/test-data/irbuild-optional.test
+++ b/mypyc/test-data/irbuild-optional.test
@@ -527,14 +527,10 @@ class B:
 
 [out]
 def f(o):
-    o :: union[object, object]
-    r0 :: object
-    r1 :: str
-    r2, r3 :: object
+    o :: object
+    r0 :: str
+    r1 :: object
 L0:
-    r0 = o
-    r1 = 'x'
-    r2 = CPyObject_GetAttr(r0, r1)
-    r3 = r2
-L1:
+    r0 = 'x'
+    r1 = CPyObject_GetAttr(o, r0)
     return 1