Maven Virtual Registry: Not allow duplicated upstreams

The following discussion from !188615 (merged) should be addressed:

• @mkhalifa3 started a discussion: (+2 comments)

  We could have duplicate upstreams in one registry. In this case, the Typhoeus::Request instance will be identical. Do you think we could utilize typhoeus' memoization to avoid sending identical requests?

🔥 Problem

Virtual Registries can be linked to a set of upstreams where two upstreams point to the the same url.

This can led to performance issues. For example, we could duplicate the request that will check the upstream.

🚒 Solution

Either

• Don't allow the same url to be used multiple times in registry.upstreams (model validation) or
• In ee/app/services/virtual_registries/check_upstreams_service.rb, remove duplicated upstreams.