What is the difference between a usd code from gox, and this code from glbse?
The difference is that gox codes are both issued and redeemed by gox. Ask gox if they would be willing to accept "gox codes" issued by someone else pursuant to an arrangement that other person made without consulting them.
Both are directly given to me. Should I give the gox code out before I hit bitinstant, I am an idiot and deserve to lose.
Of course. Nobody disputes that.
How would bitinstant deal with the same code coming in multiple times at the same time?
Bitinstant could refuse to accept any gox code they want and nobody would call them a scammer. They have no obligation to accept gox codes and they certainly don't have an obligation someone else imposed on them. They are, however, open to an issue where someone claims bitinstant redeemed a mt gox code and bitinstant claims the gox code was already redeemed. Because of this risk and others, they charge a fee for their service in exchange for which they have agreed to accept risk. They're certainly not required to accept gox codes at no charge.
First code redemption gets the credit, after that, SOL.
Say someone posts a scam accusation saying his code wasn't accepted. The response is that someone else claimed that same code first. What happens? How do we sort out which of these four things happened:
1) The same code was given to two legitimate owners.
2) The person claiming to have been refused the redemption is scamming and gave the code to someone else.
3) He's just refusing to accept the code even though nobody else claimed it.
4) Someone intercepted the code and redeemed it first.
Or, to put it more simply, if he refuses to go along with this redemption scheme, is he scamming? Honestly, if I were in this position, I would refuse to honor the codes and tell the investors to take it up with GLBSE, the party they contracted with to acquire and manage the shares which has decided to renege on the agreement. I certainly wouldn't let myself be coerced into accepting a broken redemption scheme that bears no resemblance to anything I ever agreed to do.