By the way, there is a third alternative to extending an entity and duplicating an entity. You can create your own entity not as a copy of the standard entity, but using the standard entity as a data source.
If you duplicate a standard entity, you become responsible for keeping your copy synchronized for the whole lifetime of the project. Otherwise your copy will miss bug fixes and improvements added by Microsoft to the standard entity.
Using the standard entity as a data source (rather then duplicating it) means that you don't have to maintain the code; you're simply using the standard one. There are still some cases that needs maintenance, though, such as when Microsoft add fields.
