U4-9682 Repository's have a double nested DeepCloneRuntimeCacheProvider set which means all entities are double deep cloned in and out of the cache and ensures that PreValueCollections are not also manually deep cloned.

src/Umbraco.Core/Cache/DeepCloneRuntimeCacheProvider.cs

@@ -26,6 +26,9 @@ namespace Umbraco.Core.Cache
 
         public DeepCloneRuntimeCacheProvider(IRuntimeCacheProvider innerProvider)
         {
+            if (innerProvider.GetType() == typeof(DeepCloneRuntimeCacheProvider))
+                throw new InvalidOperationException("A " + typeof(DeepCloneRuntimeCacheProvider) + " cannot wrap another instance of " + typeof(DeepCloneRuntimeCacheProvider));
+
             InnerProvider = innerProvider;
         }
 
@@ -105,9 +108,11 @@ namespace Umbraco.Core.Cache
                 var value = result.Value; // force evaluation now - this may throw if cacheItem throws, and then nothing goes into cache
                 if (value == null) return null; // do not store null values (backward compat)
 
+                //Clone/reset to go into the cache
                 return CheckCloneableAndTracksChanges(value);
             }, timeout, isSliding, priority, removedCallback, dependentFiles);
 
+            //Clone/reset to go out of the cache
             return CheckCloneableAndTracksChanges(cached);
         }
 

src/Umbraco.Core/Persistence/Repositories/DataTypeDefinitionRepository.cs

@@ -277,7 +277,7 @@ AND umbracoNode.id <> @id",
         public PreValueCollection GetPreValuesCollectionByDataTypeId(int dataTypeId)
         {
             var collection = GetCachedPreValueCollection(dataTypeId);
-            return collection == null ? null : (PreValueCollection) collection.DeepClone();
+            return collection;
         }
 
         public string GetPreValueAsString(int preValueId)

src/Umbraco.Core/Persistence/RepositoryFactory.cs

@@ -48,7 +48,12 @@ namespace Umbraco.Core.Persistence
                 _cacheHelper.IsolatedRuntimeCache.CacheFactory = type =>
                 {
                     var cache = origFactory(type);
-                    return new DeepCloneRuntimeCacheProvider(cache);
+
+                    //if the result is already a DeepCloneRuntimeCacheProvider then return it, otherwise
+                    //wrap the result with a DeepCloneRuntimeCacheProvider
+                    return cache is DeepCloneRuntimeCacheProvider 
+                        ? cache 
+                        : new DeepCloneRuntimeCacheProvider(cache);
                 };
             }