If it is a sim you originally created in CAS, in a new household, the game would have saved a blank copy in your game library. If you haven't deleted that one, that's the easiest way to start fresh.
If the sim was born in game, game generated, or added to an existing household, then this will not be the case. And, as mentioned above, creating a twin in CAS is yours best bet. In my experience CAS twins are never 100% identical. There's always a really small feature that is just slightly different. But you can either adjust that or just go with it.
