Time Machine keeping copy of drive that no longer exists

I have a problem that I can't find an answer to - Time Machine seems to be keeping a copy of an external disk that it previously backed up, but is no longer being used. The contents were moved to a different external drive which is being backed up regularly. But for some reason, each recent backup contains a copy of the backed up data of the old drive. It's not connected to the computer during the backup, it just keeps recopying it. Is this normal? How do I make it stop? It's killing the space on my Time Machine drive.

Thanks

Hopefully my explanation makes sense...