Make importing previously-removed duplicates less confusing

When reimporting (read: retagging) my library that contains a duplicate album, beet flags both versions of the album as duplicate, thus asking me twice what I want to do about it. Expected behaviour: If I remove the first version, don't flag the second version as duplicate.

Reproduce the reimport or retagging flow with a library containing a duplicate album, then trace the duplicate-handling entry point used by the CLI. Confirm how a removed first version is retained during the same operation. Done means the second version is not flagged as a duplicate and the existing duplicate prompt behavior remains correct.

Written by the indexing model from the issue text.