Unless PaO is below level 5 (6 if bloodlines are allowed), it can't be duped.
I've actually figured out an actually useful way to use this: if someone dispels one of your buffs, you can steal the other half of the buff off of your familiar.
Oh, and another question: if a Spellthief has a way of recovering spells, and they steal a spell, can they cast the spell and then recover it?
So, does anyone have any non-Master Spellthief tricks?