I believe, in this case, a STAT would determine if the order was accepted or not.
I believe STAT wouldn't be aware of packets which are in flight but haven't yet reached servers.
But this is largely a theoretical problem since you can simply wait a bit to make sure that all data in flight either arrived or died peacefully.
Also I hope that if MPex sees two identical orders it would ignore duplicate, so even if you submit twice it is not a problem.(?)
Yeah, if you submit the same data twice, it rejects the duplicate. It must be duplicate data tho, not just 'the same order'.
EDIT: so basically, don't resign a 'new' order, just resend the same signed/encrypted message as you did originally.
SECOND EDIT: This is all horribly off topic, isn't it?