Is double tapping permitted when joining 2 conductors of the same gauge into a Square D QO type breaker

No, you may not double tap a circuit breaker. (you do mean attaching two hot conductors to the same breaker, don't you?)

Apart from the NEC, it may also be prohibited by the manufacturer. It the CB has been listed as a single conductor device, UL has only listed it for that connection.

Don, please check your facts on this one.

The term "double tap" is not any kind of official term. This is a term made up by home inspectors.

There is no prohibition in the NEC to put two conductors on a breaker if that breaker is designed to accept two conductors.

Square D QO and HOM (15, 30 & 30 amp) are both designed to accept two conductors of the same size.
Many Cutler Hammer CH series are this way as well.

Stan, that's what I said.

Apart from the NEC, it may also be prohibited by the manufacturer. If the CB has been listed as a single conductor device, UL has only listed it for that connection."
"

No, you may not double tap a circuit breaker. (you do mean attaching two hot conductors to the same breaker, don't you?)

Apart from the NEC, it may also be prohibited by the manufacturer. Oh, I was going more by this, which the way I read it is saying: "No, you may not double tap a circuit breaker.".
The second sentence I thought you meant along with the NEC prohibiting it...

OK then. :)