[UDP] Can simultaneously Push multiple IAP Products with same ID and name

After installing the Universal Distribution Portal (UDP) package, "UDP Settings" will be exposed to the user, where the user can add IAP Products and "Push" them to the UDP Server. Currently, if the user batches multiple new IAP Products with the same name/IDs, they will be allowed to batch Push them to the server with no validation or error. This may result in data loss.

Steps to reproduce:
- In Unity 2019.3, create a new project and install the Universal Distribution Portal (UDP) package (alternatively download the attached project)
- Menu: Window > Universal Distribution Portal > Settings
-Inspector (UDP Settings) > Register the project with Unity Cloud Services if the UDP Settings Inspector pane indicates it is not already
- Inspector (UDP Settings) > IAP Catalog > Add new IAP
- Configure the new IAP Product with any information
- Repeat these steps to add multiple IAP Products with identical IDs/names (do not Push until finished)
- Inspector (UDP Settings) > Push
- Observe there is no error thrown and the Push appears successful
- Attempt to configure and push another new IAP Product with the same matching IDs/names
- Observe a validation error is now shown as intended: "slug: Field is not correct. Occupied by another item."