Circuit 24: has two 20 amp breakers is in one piece. Two wires are only attached to the top breaker. It seems the bottom breaker is a spare.When it trips only the top breaker trips. The 20amp top breaker serves window ac/washing machine/ refrigerator/ computer/
But they are not on, at the same time. Question is the bottom breaker just a spare?? Too bad it couldn't be use on circuit 24, to share the load?

Show us a picture of this breaker and its wiring.

Lots of reasons it was done that way.
Electrician too lazy to get the proper breaker and/or a blank could be one of them.
It's likely you can separate them and utilize both circuit, but then you have a multi wire branch circuit sharing just one neutral... and that is a maybe. Until you can figure out how everything is wired you shouldn't change anything. It actually may have been done that way for a reason... good or bad one.

Duplicate thread: https://www.askmehelpdesk.com/electrical-lighting/electrical-breaker-813317.html