The dot Poisonous Spit (503522) caused by the skill Poisonous Spit (493253) can't be stacked. So if we have two scout/rogue which cause that debuff only one will be active on the target.
On the other hand, all other dots are player dependent but not buff dependend. For example there can be several Vampire Arrows on one target at the same time.
Please change Poisonous Spit (503522) so that it works like all other dots.