did some testing this realy is a bug, while in pve its totaly unoticeable its easy to miss.
if a spell/combo received has multiple efects it will trigger a damage proc return for each efect received, so pretty much any efect placed will give a damage return proc back to caster.
should be checked into and fixed.
although it only realy is a threat to demo and sin class, its still a bug and should be fixed.