Deathmark stacks on rancor?

I tested DT under zader for the first time in rancor raid and i noticed, that when i deathmarked rancor for more times then once, it was stacking (still one deathmark icon but small bonus dmg was applied twice).
Dont know if I didnt just discover America for the second time :) , so if this is allready known fact, just ignore this threat. Hope this helps you to do better dmg :)