Yeah, I noticed this when trying to build a Parasitic Magus + Parasitic Succubus deck. In my experience, the first copy of a Parasite you discard will spawn a second copy to your hand, but you can't get any additional copies of that particular Parasite until you play one, but you CAN get more copies if you later discard it again after playing one. (Except for Parasitic Magus, haha)