Double tapping (double lugging) is the term for attaching two wires to a single fuse or breaker. Most terminals are not designed to accommodate two wires. The risk of a poor connection is greater with double taps.
Double tapping is a poor installation practice.
Some electrical inspectors will allow double taps if wires are the same size and if the connection looks good. Some authorities will accept this situation, but suggest we suggest to have it checked out. Speak to your local utility, inspectors and electricians to see what their position is.
Sometimes double taps are solved by taking the two wires that are attached to a terminal screw and removing them both. The wires are then pigtailed together with one end of a third short wire. The other end of the third wire is attached to the fuse or breaker terminal. This eliminates the double lugging, although it creates one more connection in the box.
Most utilities will accept this solution, although every additional splice is another possible poor connection. The highest quality work uses the minimum number of connections.
Sometimes double taps are accepted where the second wire is a low voltage transformer for a door bell or telephone system, for example. However, double taps are considered more problematic where the two wires are different sizes.
Some special breakers are designed to hold two wires. In this case, double taps are often permitted. To be on the safe side, you should note all double taps, and let an electrician make the final call.