Spell files not in use

I'm writing a mod that changes the descriptions of spells in BG2. It's a lot of editing because there are so many spells, but also because there seem to be many spell files which are repeated. Examples are cdstaf12.spl, OHBWI308.spl, SPCL722.spl, SPDR301.spl, SPIN579.spl, SPIN714.spl, SPWI002.spl, SPWI308.spl, SPWI399.spl and SPWI997.spl, all of which are copies of Lightning Bolt. I imagine that in all these sets of repeated files the game only uses one of the files. It would be very convenient to find out which one it uses because then I could ignore the rest, which would save me a lot of editing time. So how can I find out which files the game actually uses?