Hi there
Like all these things - until somebody has actually TRIED it the issue is still theoretical.
Now why don't you TEST it. If you are worried about the legality then you don't have to KEEP the installed system on your computer. The OP asked a technical question for which currently nobody seems to have a definitive answer. The server *Should* refuse to complete the purchase if the promo code has been used already. Note I say *Should* but it might allow the purchase to complete --up to you then to decide what to do.
My own view on this is two fold :
a) the issue is purely between "You and your Maker".
b) If the Ms servers aren't robust enough to detect what promo codes have been used and by whom then that's THEIR problem and subject to a) above either use or reject at choice.
In a way it's a bit like using a Bank ATM -- if you withdrew say 150 EUR and two 50 EUR notes were stuck together so you got 200 EUR - would you go back into the bank and return the extra 50 (might be impossible if the machine was say in a shopping Mall, Railway station etc).
I'd take a punt here that 99% of people --even if they were the most god fearing and devout people on the planet would probably opt to keep the extra 50 EUR probably thinking - we've bailed out most of these banks anyway with our taxes.
Cheers
jimbo