Player A throws an explosion potion at the NPC, and while that potion is counting down to explode, player B throws an explosion potion at player A. Now only the potion player B threw will explode, the one player A threw will remain at the NPC's feet (or wherever it was thrown), won't explode, and will be impossible to pick up.
Also - not sure how smart it is to allow people to throw multiple potions after eachother rapidly. I think we need some sort of "You must wait to throw another potion" timer