I'm returning a List (own class) from Silverlight to a service, in this list there are old entities with updated values and completely new entities, how do I save this back to the database using the entity framework?
Using cEnts.CardItems.AddObject gives me duplicates obviously.
You want to use Attach() instead of AddObject().
Attach will take your disconnect object and let the container know to consider it for updates. The new objects, without a PrimaryKey, will be added.