diff --git a/src/code-stub-assembler.cc b/src/code-stub-assembler.cc
index a49ae86e52..3bcd6f1c28 100644
--- a/src/code-stub-assembler.cc
+++ b/src/code-stub-assembler.cc
@@ -3085,6 +3085,24 @@ TNode<MutableHeapNumber> CodeStubAssembler::AllocateMutableHeapNumber() {
   return UncheckedCast<MutableHeapNumber>(result);
 }
 
+TNode<Object> CodeStubAssembler::CloneIfMutablePrimitive(TNode<Object> object) {
+  TVARIABLE(Object, result, object);
+  Label done(this);
+
+  GotoIf(TaggedIsSmi(object), &done);
+  GotoIfNot(IsMutableHeapNumber(UncheckedCast<HeapObject>(object)), &done);
+  {
+    // Mutable heap number found --- allocate a clone.
+    TNode<Float64T> value =
+        LoadHeapNumberValue(UncheckedCast<HeapNumber>(object));
+    result = AllocateMutableHeapNumberWithValue(value);
+    Goto(&done);
+  }
+
+  BIND(&done);
+  return result.value();
+}
+
 TNode<MutableHeapNumber> CodeStubAssembler::AllocateMutableHeapNumberWithValue(
     SloppyTNode<Float64T> value) {
   TNode<MutableHeapNumber> result = AllocateMutableHeapNumber();
@@ -5027,7 +5045,8 @@ void CodeStubAssembler::CopyPropertyArrayValues(Node* from_array,
                                                 Node* to_array,
                                                 Node* property_count,
                                                 WriteBarrierMode barrier_mode,
-                                                ParameterMode mode) {
+                                                ParameterMode mode,
+                                                DestroySource destroy_source) {
   CSA_SLOW_ASSERT(this, MatchesParameterMode(property_count, mode));
   CSA_SLOW_ASSERT(this, Word32Or(IsPropertyArray(from_array),
                                  IsEmptyFixedArray(from_array)));
@@ -5039,9 +5058,14 @@ void CodeStubAssembler::CopyPropertyArrayValues(Node* from_array,
   ElementsKind kind = PACKED_ELEMENTS;
   BuildFastFixedArrayForEach(
       from_array, kind, start, property_count,
-      [this, to_array, needs_write_barrier](Node* array, Node* offset) {
+      [this, to_array, needs_write_barrier, destroy_source](Node* array,
+                                                            Node* offset) {
         Node* value = Load(MachineType::AnyTagged(), array, offset);
 
+        if (destroy_source == DestroySource::kNo) {
+          value = CloneIfMutablePrimitive(CAST(value));
+        }
+
         if (needs_write_barrier) {
           Store(to_array, offset, value);
         } else {
@@ -5050,6 +5074,18 @@ void CodeStubAssembler::CopyPropertyArrayValues(Node* from_array,
         }
       },
       mode);
+
+#ifdef DEBUG
+  // Zap {from_array} if the copying above has made it invalid.
+  if (destroy_source == DestroySource::kYes) {
+    Label did_zap(this);
+    GotoIf(IsEmptyFixedArray(from_array), &did_zap);
+    FillPropertyArrayWithUndefined(from_array, start, property_count, mode);
+
+    Goto(&did_zap);
+    BIND(&did_zap);
+  }
+#endif
   Comment("] CopyPropertyArrayValues");
 }
 
diff --git a/src/code-stub-assembler.h b/src/code-stub-assembler.h
index 767d3969fa..e6f64ff4fb 100644
--- a/src/code-stub-assembler.h
+++ b/src/code-stub-assembler.h
@@ -1589,10 +1589,19 @@ class V8_EXPORT_PRIVATE CodeStubAssembler : public compiler::CodeAssembler {
                                       Node* to_index,
                                       ParameterMode mode = INTPTR_PARAMETERS);
 
-  void CopyPropertyArrayValues(
-      Node* from_array, Node* to_array, Node* length,
-      WriteBarrierMode barrier_mode = UPDATE_WRITE_BARRIER,
-      ParameterMode mode = INTPTR_PARAMETERS);
+  enum class DestroySource { kNo, kYes };
+
+  // Specify DestroySource::kYes if {from_array} is being supplanted by
+  // {to_array}. This offers a slight performance benefit by simply copying the
+  // array word by word. The source may be destroyed at the end of this macro.
+  //
+  // Otherwise, specify DestroySource::kNo for operations where an Object is
+  // being cloned, to ensure that MutableHeapNumbers are unique between the
+  // source and cloned object.
+  void CopyPropertyArrayValues(Node* from_array, Node* to_array, Node* length,
+                               WriteBarrierMode barrier_mode,
+                               ParameterMode mode,
+                               DestroySource destroy_source);
 
   // Copies all elements from |from_array| of |length| size to
   // |to_array| of the same size respecting the elements kind.
@@ -3213,6 +3222,10 @@ class V8_EXPORT_PRIVATE CodeStubAssembler : public compiler::CodeAssembler {
 
   TNode<JSArray> ArrayCreate(TNode<Context> context, TNode<Number> length);
 
+  // Allocate a clone of a mutable primitive, if {object} is a
+  // MutableHeapNumber.
+  TNode<Object> CloneIfMutablePrimitive(TNode<Object> object);
+
  private:
   friend class CodeStubArguments;
 
diff --git a/src/ic/accessor-assembler.cc b/src/ic/accessor-assembler.cc
index 2c8684fe2c..b677ab31c5 100644
--- a/src/ic/accessor-assembler.cc
+++ b/src/ic/accessor-assembler.cc
@@ -1684,7 +1684,8 @@ Node* AccessorAssembler::ExtendPropertiesBackingStore(Node* object,
     // |new_properties| is guaranteed to be in new space, so we can skip
     // the write barrier.
     CopyPropertyArrayValues(var_properties.value(), new_properties,
-                            var_length.value(), SKIP_WRITE_BARRIER, mode);
+                            var_length.value(), SKIP_WRITE_BARRIER, mode,
+                            DestroySource::kYes);
 
     // TODO(gsathya): Clean up the type conversions by creating smarter
     // helpers that do the correct op based on the mode.
@@ -3620,7 +3621,7 @@ void AccessorAssembler::GenerateCloneObjectIC() {
       auto mode = INTPTR_PARAMETERS;
       var_properties = CAST(AllocatePropertyArray(length, mode));
       CopyPropertyArrayValues(source_properties, var_properties.value(), length,
-                              SKIP_WRITE_BARRIER, mode);
+                              SKIP_WRITE_BARRIER, mode, DestroySource::kNo);
     }
 
     Goto(&allocate_object);
@@ -3640,7 +3641,8 @@ void AccessorAssembler::GenerateCloneObjectIC() {
     BuildFastLoop(source_start, source_size,
                   [=](Node* field_index) {
                     Node* field_offset = TimesPointerSize(field_index);
-                    Node* field = LoadObjectField(source, field_offset);
+                    TNode<Object> field = LoadObjectField(source, field_offset);
+                    field = CloneIfMutablePrimitive(field);
                     Node* result_offset =
                         IntPtrAdd(field_offset, field_offset_difference);
                     StoreObjectFieldNoWriteBarrier(object, result_offset,
diff --git a/test/mjsunit/es9/object-spread-ic.js b/test/mjsunit/es9/object-spread-ic.js
index d76ffd4eb8..55d60f2cf8 100644
--- a/test/mjsunit/es9/object-spread-ic.js
+++ b/test/mjsunit/es9/object-spread-ic.js
@@ -99,3 +99,25 @@
   // Megamorphic
   assertEquals({ boop: 1 }, f({ boop: 1 }));
 })();
+
+// There are 2 paths in CloneObjectIC's handler which need to handle double
+// fields specially --- in object properties, and copying the property array.
+function testMutableInlineProperties() {
+  function inobject() { "use strict"; this.x = 1.1; }
+  const src = new inobject();
+  const x0 = src.x;
+  const clone = { ...src, x: x0 + 1 };
+  assertEquals(x0, src.x);
+  assertEquals({ x: 2.1 }, clone);
+}
+testMutableInlineProperties()
+
+function testMutableOutOfLineProperties() {
+  const src = { a: 1, b: 2, c: 3 };
+  src.x = 2.3;
+  const x0 = src.x;
+  const clone = { ...src, x: x0 + 1 };
+  assertEquals(x0, src.x);
+  assertEquals({ a: 1, b: 2, c: 3, x: 3.3 }, clone);
+}
+testMutableOutOfLineProperties();