Detect multiple domains pointing to a same unique instance

[joenepraat]

When someone makes an redirect to an instance (and probably adds this one to instances.social/admin) it is shown on instances.social and joinmastodon.org.

Example:

instances.social

joinmastodon.org

The first instance is the correct one, the second the redirect.

[TheKinrar]

Although joinmastodon.org does not use instances.social API anymore, and cat.social also died since, it would be a great idea to detect duplicates instances with multiple domains.