A double tap is when two or more wires are attached to a single circuit breaker.
In most cases this isn’t allowed, because the circuit breaker is designed to accept only one wire. So if two or more wires are attached then they might not all be making good electrical contact, and that can cause an electrical arc and overheating so it’s a fire hazard. Any good electrician knows that a double tap isn’t allowed, so this is very poor workmanship and possibly means that the work wasn’t done by an electrician but by a handyman or homeowner. That’s a problem because you don’t know what else he did wrong.
Some circuit breakers do allow two wires to be attached, and a good electrician knows when this is the case. Keep in mind that you always have to follow the installation instructions from the manufacturer, so if the manufacturer says that only one wire is allowed then that’s the rule no excuses.
There are several options for fixing a double tap, all of which should be done by a good, qualified electrician.
The simplest option is to use a pigtail. Remove both wires from the circuit breaker, and attach a single short piece of wire to the breaker. Then take the two original wires and connect them together with the short pigtail wire, using a wire nut. This is pretty simple and will always be acceptable (assuming all the other wiring is correct). After all, most electrical circuits split off somewhere, so you might as well do it here. This works because it follows the rules: there’s only one wire attached to the breaker, the wire nut is sized to accept all three of these wires, and this short pigtail wire is big enough to safely carry all of the current that the breaker is rated for (15 amps in this case).
Another option is to use one of the breakers that allows two wires. These circuit breakers are always clearly labelled with the number, size, and type (copper or aluminum) wires that can be connected.
This breaker pictured below allows one aluminum or copper wire from 14 AWG (American Wire Gauge) to 8 AWG, or two copper wires from 14 to 10 AWG (the smaller the gauge number the bigger the wire).
The problem with this solution is that you might not be able to find a breaker that fits your electrical panelboard and that accepts two wires.
Another option is to install an additional circuit breaker into your electrical panel and move one of the wires over to it. This is a great solution, but there might not be any more space for additional breakers.
The last option is to use a tandem circuit breaker. This is a device that fits two circuit breakers into the space normally used for one breaker. The problem here is that some electrical panels don’t allow tandem breakers or they restrict how many and where they can be installed. So there’s no guarantee that you can find one that fits your electrical panelboard and there’s no guarantee that there are more spaces for another tandem breaker.
With all of these options you need to be sure that the circuit’s neutral wire is dealt with correctly. That and general safety concerns mean that you should have a good qualified electrician performing this work.