We see them fairly often; breakers designed for one conductor but that have two or more conductors inserted, called a "double-tap".
The concern is that when two conductors are used in a lug designed for only one, a poor connection may exist. Poor connections are prone to overheating, a potential fire hazard.
Some manufacturers produce breakers designed for two conductors. Some examples are: Square-D QO and Homeline panels, using 15-30 amp non-GFCI/AFCI breakers, and some Cutler Hammer CH panels (except bedroom circuits).
Like those in the photo above, they have tabs designed to hold two conductors (but not three)...
and the breaker will typically be labeled. Looking at the symbols in the photo above, two copper conductors (CU) can be attached, but only one aluminum conductor (ALCU).
Labels specifying two conductors may vary with different manufacturers.
Both Square-D and Cutler Hammer do allow copper conductors #14 to #10 AWG to be installed.
THE FIX
Attaching two conductors to a breaker designed for only one is a potential fire hazard, but this condition doesn't reduce the ability of a breaker to protect a circuit. Bearing this in mind, the fix is fairly simple. Two conductors are pigtailed into one, which is then connected to the breaker...
as seen in the photo above.