Hello! Came back to say I found a new bug, one that I'm not sure about the specifics of how it works, but that I've managed to replicate reliably. Sorting spells in the inventory, if there is an empty space in the same spot for weapons that you put the first spell, will place the spell into the weapons inventory. If you try to move it to another empty spot from there, it will duplicate.
You can regain the spells as spells proper by putting them into storage and retrieving them, but, somehow related to this, is the fact that I also bought a spell and didn't receive it. I'm not sure if it's related to this at all, but looking for it to use (Heal II) was how I found out about this issue being reliably re-creatable, as I'd once done it before by accident with an item.