Two 12 gauge wires hooked into a single 20A Breaker whats up with that?

It called stacking and its actually common Manufactures used to even make a provision to put two wires on there breakers .But its not a good idea to stack wires on breakers Is there room for another breaker if not you should get a TANDEM breaker its two breakers in the size of a standard breaker But there only single pole So remember that alot of people fil there panel with these breakers sometimes depending on the panel type they load one side with the tandem causing unbalanced loads But in your case it should be okay to use a tandem Just get the exact same model type that are there now , same amperage its easy and al there will be is 2 wires to hook up .But if you have a opening in the dead front cover that you can use do that instead of the tandem breaker Leaving only one wire to connect either way its fixable and should not be difficult or expensive to fix But your correct some lazy a$$ nitwit did exactly that connected to first breaker he saw called it done ( only to his feeble standards ) and got paid , and left .. i seriously don't like that type of person / hack myself

Sounds like complete laziness/cheapness on whoever hooked that up.

You did the correct thing, and 12 gauge is correct for a 20 amp circuit.

ya, that's exactly what they did, but its possible it was justified. code says on a 12 g. breaker, you can have 10 receptacles and 3 lights. so, if the dude was checkin, it may have been ok, except that all electricians know that a microwave is suppose to be on its own dedicated breaker. good job!!

Thanks guys I figured that's what was up but just surprised that a professional electrician would do that especially with 10 empty slots still in the panel.