One option you might have is to add Catalog Manager as an Approver to the Subscription, and he will need to validate if the activating Subscription is required to be added as a catalog to buyer, or if it is a duplicate entry and other negotiation.
The Ariba Network and the Buyer application both support versioning of the catalog. The supplier has to version the existing catalog on the Network and then the Buyer application with Catalog Subscription Sync will show you a report of what items have been added, deleted or changed.
The problem looks like the supplier is giving the uploaded catalog a completely different name and therefore the system thinks it is a different catalog. Try having the supplier open an existing catalog and then on the contents area click on Create New Version. This should give you what you need.