According to the Star Tribune, a double tap occurs when two conductors are connected under one screw inside a panelboard. Most circuit breakers do not support double tapping, although some manufacturers, such as like Cutler Hammer, make hardware specially designed for this purpose.
Double tapping is a defect when it is used on incompatible devices. If the conductors come loose, they cause overheating and electrical arcing, and the risk of fire is also present. A double tap can be accommodated by installing a new circuit board compatible with double tapping. It is also possible to add another circuit breaker or install a tandem breaker to the existing breaker box.