Elementary clone
You were most likely reflecting the mesmer’s projectiles from his/her scepter with your Magnetic Aura (earth → staff 3). This actually works with any type of reflect and is caused by the fact, that with every 3rd hit in the mesmer’s scepter auto hit chain, a clone is being produced on hit.
Since you were the one hitting the mesmer with said projectile, you created a clone of yourself.
I think it’s weird, but funny.
Thanks to this answer.
It can be done with loads of stuff, if you had pop’d tornado at the right time it would have cloned the tornado too ^.^