The API should not allow for adding duplicate assets to products

Description

This is a fix for an issue where the API allows for addition of duplicate assets to products.

Scenario: When adding multiple media items with the same key/ID to a product, it causes an exception (duplicate key) when saving the next time.

Cause: 

AssetServiceDatabase.CommitAssets does not check duplication for the updatingRows list.

In this case, an exception should be thrown.