What's the best way to resolve multiple circuits going to a single outlet?
Based on your description, I would disconnect the outlet from circuit 10. That way everything electrical you can see (switches, receptacles, ceiling light/fan, etc.) is connected to circuit 1, in a way that makes sense.
You'll need to attach wire caps to circuit 10 so it's safe. Unfortunately the only way to remove that circuit properly is to cut open the wall, which is a much bigger mess.