Detecting duplicates

[Zlopez]

Here is an idea for detecting duplicates in release-monitoring.org.

Each backend now has get_backend_url method which gives us a URL for the project that is actually checked for the new versions. As this method always returns same URL for different Anitya projects that check same project upstream we can leverage that for some kind of duplicate detection, when the project is created and for finding duplicates in existing projects as well.

Cleaning the existing projects will be probably just a one-shot process after this will be introduced to project creation.

We will still have duplicates for projects with different backends, but at least it will allow us to quickly detect duplicate projects with same backend.