My experience is to do for each promotion even if it's for the same product. What differentiates the promotion is the mail-in address or the promotion name on the mail-in addressee. As long as the mail-in Promotion Offer or PO Box is different, it's a different promotion. I try to do them all....
Added:
Also, if the rebate is for same product but for different dates, they are considered different promotions and not duplicates. I do those as well.