Most breakers are designed to have only one hot wire connected to them. Basically one circuit to one breaker is the general rule. Naturally, as always, there are a few exceptions and this is when an electrician should be consulted.
One of the concerns is that if a breaker is not designed and approved for two hot wires, then one of the wires may be or become a little loose. This happens overtime form the breaker heating up and then cooling off repeatedly. Then, occasionally, electrical arcing or over heating may occur.
Easy to correct
Often this can be easily corrected by adding another breaker and moving one of the wires over to the added breaker. When there is no more room for another breaker in the panel, then there may be a special breaker that is designed for the one slot, but will hold two circuits. In rare occasions a small sub panel may be added if the main panel does not have extra space or an existing breaker in the main panel can’t be replaced with one that can handle two circuits.
Double tapping is one of the more common findings at a home inspection.