See this is my worry, if they can identify each individual disc, and each coupon coupon is obviously individual, then maybe they each coupon is only redeemable on this disc it was boxed with. Or maybe each disc only allows one activation of the code, irregardless of wether it was the one in the box. Or maybe it's as you said, each code can be activated just once, but on any disc not matter how many codes have been redeemed with that disc.
Argh! Maybe I'll just buy it new again, it's been out just long enough too be starting to get discounted, but not quite long enough to be cheap pre-owned. Plus, an extra sale to Valve is fine with me.