Codes can be "tagged " or "black-listed".Microsoft does that too.And some other older games,but I don't think it is still done these days.If a code is found to be leeked or used more times it is deleted from the viable data-base and if you have that code you will probably have to buy a new one ... (happened to me on steam once ,had to pay +10$ to get some CD-key for my HL2-GOTY edition ,there was some weird problem some time ago,looks like all the players that got GOTY ,including me ,had the same problem with cd-keys not being accepted.However,Valve gave us the money back ,and gave us CS-Condition-zero as a way of saying "I'm sorry" ).
For SH3 ,cD-keys don't seem to be so important,as some games don't even require them ,some hare duplicates...but for games like COD and COD2 or ... Homeworld ... Swat4 and others ,when 2 people are on the net at the same time with the same cd-key ,they both can't play anything.I'd think I'd be very pissed if this happened to me because someone used a generator and got my key .