Ever since NGS different release events are usually different releases. I think it would be sensible to give the user a hint if the medium they attempt to attach a TOC to already has a disc ID so they can double check their CD really is represented by that release.

-A warning when a disc ID is already attached to another medium on the same release could also help prevent mistakenly adding CD TOCs to the wrong medium if track counts are equal. To clarify, this is for situations where one has already passed the CD lookup results page, e.g. moving a disc ID-
Maybe scratch that. Have to think this through.