We get many calls for the correction of double tapped breakers. What this means is two wires are under a screw of a breaker that is only rated for one wire.
There are 2 ways to correct this. If the circuit is able to hold the existing wiring to the breaker then one can wire nut the 2 wires together, pigtail, so that you have one wire going to the breaker. The other is to desperate them and install a second breaker. Adding the second breaker is the best method as it lightens the loads on each circuit so the one doesn’t have the chance of being overloaded. In many old panels there is not enough room to add the second breaker thus pigtail the two circuits together is the only option.