Dunno, I know enough duplicate exploits in games to know giving the effect then reducing the item, is a pretty common source of duplication hacks/bugs.

I guess it comes down to which is the designer is more afraid of happening, the chance of a wish being expended but not granted, or granted without expending.

Then again based on disney's aladin, tricking the genie into rescuing him without using a wish, it does seem practical to assume that the genie errors on the side of granting without expending.