Here is an excerpt. The bold is mine, is that accurate?
"In determining when double-tapping is or is not acceptable for a particular circuit breaker, a simple rule of thumb is to check the design of the connecting hardware at the breaker. If the hardware is specifically shaped to accommodate two separate wires, as with Square D type QO breakers, then the connection is acceptable and should not be cited as double-tapping in a home inspection report. But if the connecting hardware is a simple screw or lug, it is reasonable to assume that the manufacturer of the breaker intended there to be one wire only at the connection. In that case, double-tapping would be the proper disclosure for a home inspector. The only way to connect two circuits to a single breaker in that instance would be by indirect means. The accepted method would be to connect a short wire (known as a “pigtail”) to the breaker and to join the other end of that wire to the two circuit wires with an appropriate connector, such as a wire nut. "
Pigtails are acceptable, but there is also too much of a good thing. I have seen panels with 10 or 12 pigtails. In that case, it is close to the time that a new and larger panel be installed.
Some local municipalities (like Chicago and some surrounding towns) to not, by local code, allow double taps even if the breaker is designed for it. Chicago also does not allow mini breakers (two breakers on one lug) even if the manufacturer does.