If your "clues" are correct, how are you going to hadle it when someone not knowing what goes on send you an illegal board?
Not yet thought of what to do in that event what do you suggest?

On that one, you need to be proactive - send out a letter to all customers stating something like "it has come to our attention that bnootleg copies of out product has been made. We suggest that you be careful that when you buy additional units you get the genuine thing. We can, of course, not guarantee anything about the performance of such bootleg products"

Whats the probability that the two consequtive reads read diffrent serial?
I do not know, I did it the first time I used the chip, the why is forgotten. Anyhow it is no big thing to do it twice and "better safe than sorry"

Erik