I think what is the problem. Playing with a thief usually i try to finish a enemy using first cloack and dagger who makes me invisible. But if i hit the button twice by mistake the spell activate twice and i lose the invi.
So i make the conclusion that if you are a heartseeker spamer , you hit someone spamming the button and the enemy recive one hit being visible and then became invisible, the accion of the skill will be repeated the same times you hit the button considering the enemy as a visible so the enemy with recive all the damage.
Is like there is no time between the hit of the key and the skill action .
Sorrry for my english… i hope you understand the concept.