I mentioned this once when on the server, but I thought I'd share here what I've found out about duplicating headcrabs.
Fast headcrabs seem to be the only ones that duplicate, and it seems to happen once you pick up a normal or poison headcrab. Happens pretty much all the time.
I don't know if knowing how to recreate it will help you locate the bug at all, but at least it's reported on the forums now.