Handle interwiki edge case

Be less surprised if the interwiki table is corrupted.

Also simplifies some defensive logic, we don't actually care if external hosts
are repeated with different URLs. This would create a lot of log spam in
production, since we already know that several of these interwiki entries exist.

Bug: T225515
Change-Id: Id51c5a5ed6a52354d4d62eb2c667aa850f9a8fb9