Suppose you don't remember that the tag is already assigned and will expect it to appear. It not appearing will cause irritation. So I think this there's no great harm in listing tags that are already assigned.

Confirmed, will be fixed soon!

Sorry Andrew but your suggestion came in a bit too late to change the implementation.
But even with the current implementation you can get what you want if you search by tag names.

It's used for Note Properties now ;) But Shift+Enter is not much worse IMO ;)

I agree) Now it's a feature request)