An existing post regarding this issue:
I’d like to note that it is not only sorcers that can do this. And you can do this with 0 dex.
It would seem that any projectile ability of the mage can replicate this bug, with me being a warlock, I have tested some abilities as seen at the GIFs and video below:
On the last GIF, the Missile were also “duped” in the process.
Adding from the other post, it is noted that other players in fact do not see this bug.
I have asked around people while I do it, and they replied that they only see one bomb, and the other ability used.
For the damage, I am yet to capture it on video, as I am currently having a hard time finding a solo mob. This also happens not only to recreate the damage of the bomb, but also the effects of the bomb, such as the Venom Bomb.