Yes, absolutely. Sea anemonaes (Anthozoans) routinely engage in "chemical warfare" with their nematocysts. The poison is toxic to any animal; the only reason cnidarians don't poison themselves with it is because their nematocysts don't leak the venom into the rest of their bodies.

This is a cool question, with a very cool answer. Yes, the nematocysts of cnidarians can sting you, other cnidarians and even themselves. Often times the cnidarians will fight with others for territory on a reef by stinging.

I know that at least some cnidarians do sting each other. Sea anemones cling to rocks and sting small fish and other prey. They can reproduce asexually by splitting themselves in two, producing clones. (They can also reproduce sexually.) These clones never sting each other, but they can have territorial clone wars with other "families." The anemones on the outer edges of these groups whack each other with their tentacles and sting each other. Stinging has a cost, because a nematocyst can only be used once. It makes sense for some anemones to sacrifice themselves because all the other individuals in their clone will pass on their genes.

Why do you think they only reproduce by cloning some of the time? Why not always clone them?