A voodoo doll was played against me and, in the interest of learning whether or not it would work, I put a purify on him. It didn't affect me, which is a bummer, since poisons WILL affect us both. But what is worse: when the voodoo doll died, I lost the OTHER purify that I had on myself.
That is to say, I had a +2 purify on myself, then they made a voodoo doll, I put another purify on it, but it didn't stack. Then, when I killed the doll (shot it with an arrow), it removed the +2 purify affect that I had on me.
So either it should stack, or it shouldn't affect me when it dies, but either way I think it is safe to call this one a bug.