That depends on how the seeds are generated. But I am sure it would be random and not selected from a list. And the probability of having the same seeds randomly is almost 0.
Almost 0 doesnt mean that it is imposible because there is still a posibillity for that. He might have been lucky enough to notice it and exploit it further. Although this all is a pure speculations and theories on how he did it, it hasnt been proven yet that two account rolling with the same seed could generate the same server seed.
Well every is a theory and impossible until it is proven so and we had no idea on how he did this so what we are doing right now are speculating on something that might come to sense.
The flaw could mean anything, more interesting is that on how he was able to find that there is such flaw that will reveal the seed on the site
By what you meant as duplicated account , do you mean that he have 2 account with the same username? I dont think this is kinda possible because each sites got their own system restriction whenever someone is trying to register with the same username though
I think hufflepuff somehow duplicated same UID on 2 different accounts and therefor he was able to reveal the seed on 1 without affecting the other one
Still not possible to have two account with the same UID though. Well if this is the case then he would have done it by keep on sending request to the server to make a new account but it will need a constant retry of doing this and not to mention the near zero possibilities of creating it
not if the 2 accounts were created same time