diff --git a/Code/Framework/AzCore/AzCore/Serialization/Json/MapSerializer.cpp b/Code/Framework/AzCore/AzCore/Serialization/Json/MapSerializer.cpp
index 196ce28216..f7e41c0c00 100644
--- a/Code/Framework/AzCore/AzCore/Serialization/Json/MapSerializer.cpp
+++ b/Code/Framework/AzCore/AzCore/Serialization/Json/MapSerializer.cpp
@@ -203,7 +203,7 @@ namespace AZ
     JsonSerializationResult::Result JsonMapSerializer::LoadElement(void* outputValue, SerializeContext::IDataContainer* container,
         const SerializeContext::ClassElement* pairElement, SerializeContext::IDataContainer* pairContainer,
         const SerializeContext::ClassElement* keyElement, const SerializeContext::ClassElement* valueElement,
-        const rapidjson::Value& key, const rapidjson::Value& value, JsonDeserializerContext& context)
+        const rapidjson::Value& key, const rapidjson::Value& value, JsonDeserializerContext& context, bool isMultiMap)
     {
         namespace JSR = JsonSerializationResult;
 
@@ -231,8 +231,30 @@ namespace AZ
             return context.Report(keyResult, "Failed to read key for associative container.");
         }
 
+        void* valueAddress = nullptr;
+        bool keyExists = false;
+
+        // For multimaps, we append values to keys instead updating them.
+        // This is to ensure legacy multimap serialization support.
+        if (!isMultiMap)
+        {
+            auto associativeContainer = container->GetAssociativeContainerInterface();
+            void* existingKeyValuePair = associativeContainer->GetElementByKey(outputValue, keyElement, keyAddress);
+            if (existingKeyValuePair)
+            {
+                valueAddress = pairContainer->GetElementByIndex(existingKeyValuePair, pairElement, 1);
+                expectedSize--;
+                keyExists = true;
+            }
+        }
+
+        // If the key doesn't exist or it's a multimap, we're adding the new element we reserved above.
+        if (!keyExists)
+        {
+            valueAddress = pairContainer->GetElementByIndex(address, pairElement, 1);
+        }
+
         // Load value
-        void* valueAddress = pairContainer->GetElementByIndex(address, pairElement, 1);
         AZ_Assert(valueAddress, "Element reserved for associative container, but unable to retrieve address of the value.");
         ContinuationFlags valueLoadFlags = ContinuationFlags::LoadAsNewInstance; 
         if (valueElement->m_flags & SerializeContext::ClassElement::Flags::FLG_POINTER)
@@ -257,7 +279,18 @@ namespace AZ
         }
         else
         {
-            container->StoreElement(outputValue, address);
+            // Even if the key exists, calling StoreElement will not replace the existing key
+            // and will free the temporary address as expected. Checking if the key already 
+            // exists and skipping the call to StoreElement if it does, makes the intent more
+            // clear. The end result is the same either way.
+            if (!keyExists)
+            {
+                container->StoreElement(outputValue, address);
+            }
+            else
+            {
+                container->FreeReservedElement(outputValue, address, context.GetSerializeContext());
+            }
             if (container->Size(outputValue) != expectedSize)
             {
                 return context.Report(JSR::Tasks::ReadField, JSR::Outcomes::Unavailable,
@@ -430,7 +463,7 @@ namespace AZ
     JsonSerializationResult::Result JsonUnorderedMultiMapSerializer::LoadElement(void* outputValue, SerializeContext::IDataContainer* container,
         const SerializeContext::ClassElement* pairElement, SerializeContext::IDataContainer* pairContainer,
         const SerializeContext::ClassElement* keyElement, const SerializeContext::ClassElement* valueElement,
-        const rapidjson::Value& key, const rapidjson::Value& value, JsonDeserializerContext& context)
+        const rapidjson::Value& key, const rapidjson::Value& value, JsonDeserializerContext& context, [[maybe_unused]] bool isMultiMap)
     {
         namespace JSR = JsonSerializationResult;
 
@@ -440,7 +473,7 @@ namespace AZ
             for (auto& entry : value.GetArray())
             {
                 result.Combine(JsonMapSerializer::LoadElement(outputValue, container, pairElement, pairContainer,
-                    keyElement, valueElement, key, entry, context));
+                    keyElement, valueElement, key, entry, context, true));
                 if (result.GetProcessing() == JSR::Processing::Halted)
                 {
                     return context.Report(result, "Unable to process the key or all values in multi-map.");
@@ -451,7 +484,7 @@ namespace AZ
         else if (IsExplicitDefault(value))
         {
             return JsonMapSerializer::LoadElement(outputValue, container, pairElement, pairContainer,
-                keyElement, valueElement, key, value, context);
+                keyElement, valueElement, key, value, context, true);
         }
         else
         {
diff --git a/Code/Framework/AzCore/AzCore/Serialization/Json/MapSerializer.h b/Code/Framework/AzCore/AzCore/Serialization/Json/MapSerializer.h
index 937c8389ff..74d7e81ac9 100644
--- a/Code/Framework/AzCore/AzCore/Serialization/Json/MapSerializer.h
+++ b/Code/Framework/AzCore/AzCore/Serialization/Json/MapSerializer.h
@@ -32,7 +32,7 @@ namespace AZ
         virtual JsonSerializationResult::Result LoadElement(void* outputValue, SerializeContext::IDataContainer* container, 
             const SerializeContext::ClassElement* pairElement, SerializeContext::IDataContainer* pairContainer, 
             const SerializeContext::ClassElement* keyElement, const SerializeContext::ClassElement* valueElement, 
-            const rapidjson::Value& key, const rapidjson::Value& value, JsonDeserializerContext& context);
+            const rapidjson::Value& key, const rapidjson::Value& value, JsonDeserializerContext& context, bool isMultiMap = false);
         
         virtual JsonSerializationResult::Result Store(rapidjson::Value& outputValue, const void* inputValue, const void* defaultValue,
             const Uuid& valueTypeId, JsonSerializerContext& context, bool sortResult);
@@ -62,7 +62,7 @@ namespace AZ
         JsonSerializationResult::Result LoadElement(void* outputValue, SerializeContext::IDataContainer* container,
             const SerializeContext::ClassElement* pairElement, SerializeContext::IDataContainer* pairContainer,
             const SerializeContext::ClassElement* keyElement, const SerializeContext::ClassElement* valueElement,
-            const rapidjson::Value& key, const rapidjson::Value& value, JsonDeserializerContext& context) override;
+            const rapidjson::Value& key, const rapidjson::Value& value, JsonDeserializerContext& context, bool isMultiMap = false) override;
 
         using JsonMapSerializer::Store;
         JsonSerializationResult::Result Store(rapidjson::Value& outputValue, const void* inputValue, const void* defaultValue,
diff --git a/Code/Framework/AzCore/Tests/Serialization/Json/MapSerializerTests.cpp b/Code/Framework/AzCore/Tests/Serialization/Json/MapSerializerTests.cpp
index 7ffe3ffee0..fe97d36f08 100644
--- a/Code/Framework/AzCore/Tests/Serialization/Json/MapSerializerTests.cpp
+++ b/Code/Framework/AzCore/Tests/Serialization/Json/MapSerializerTests.cpp
@@ -475,7 +475,7 @@ namespace JsonSerializationTests
         EXPECT_STRCASEEQ("value_42", worldKey->second.m_value.c_str());
     }
 
-    TEST_F(JsonMapSerializerTests, Load_DuplicateKey_EntryIgnored)
+    TEST_F(JsonMapSerializerTests, Load_DuplicateKey_EntryUpdated)
     {
         using namespace AZ::JsonSerializationResult;
 
@@ -489,12 +489,12 @@ namespace JsonSerializationTests
         StringMap values;
         ResultCode result = m_unorderedMapSerializer.Load(&values, azrtti_typeid(&values), *m_jsonDocument, *m_jsonDeserializationContext);
 
-        EXPECT_EQ(Processing::PartialAlter, result.GetProcessing());
-        EXPECT_EQ(Outcomes::Unavailable, result.GetOutcome());
+        EXPECT_EQ(Processing::Completed, result.GetProcessing());
+        EXPECT_EQ(Outcomes::Success, result.GetOutcome());
 
         auto entry = values.find("Hello");
         ASSERT_NE(values.end(), entry);
-        EXPECT_STRCASEEQ("World", entry->second.c_str());
+        EXPECT_EQ("Other", entry->second);
     }
 
     TEST_F(JsonMapSerializerTests, Load_DuplicateMultiKey_LoadEverything)
@@ -536,8 +536,8 @@ namespace JsonSerializationTests
         ResultCode result = m_unorderedMapSerializer.Load(&values,
             azrtti_typeid(&values), *m_jsonDocument, *m_jsonDeserializationContext);
 
-        EXPECT_EQ(Processing::Altered, result.GetProcessing());
-        EXPECT_EQ(Outcomes::Unavailable, result.GetOutcome());
+        EXPECT_EQ(Processing::Completed, result.GetProcessing());
+        EXPECT_EQ(Outcomes::Success, result.GetOutcome());
 
         auto entry = values.find("Hello");
         ASSERT_NE(values.end(), entry);