Can you tell if a breaker can be double tapped by looking at the terminal?
In my opinion the particular breaker terminal you are showing, is legal for connection of two conductors. The only residential breaker that I know of that has this feature is a Square D Type QO. Maybe a Cutler Hammer Type CH, I think? 

Regardless, it doesn't matter what my opinion is unless it is the AHJ agrees. I have had this discussion with local Inspectors many times and I can say that I have won about 50% of the time and lost 50% of the time.

In conclusion, check with your local authorities because what he allows and doesn't allow is really all that matters.