druids have 2 sorts of dots. one is the stinging swarm line and the second is the fire based dot with an AC debuff in it.
The fire based dot does not stack. Its because of the AC debuff.
The stinging swarm dot does stack. I can stack all upgrades of the spell on eachother. and if i'm grouped with another druid. that druid can cast all his dots of that spell line on the mob as well.
But this is just how it should be. You can read comments about it here:
http://everquest.allakhazam.com/db/spell.html?spell=259