Yikes! Thanks for the report. That's a pretty big bug! Could you please give a step by step on how you recreated it? I'm trying to do as you say but don't seem to be getting any duplication issues.
When you say you bought a spell and couldn't find it, was this before or after you used this item duplication bug? Also, which shop was you trying to buy Heal II from?
EDIT: Okay. I've located and fixed the item duplication issue but I haven't been able to replicate the issue of a purchased spell not showing up in the spells menu.