If you had it replaced via RMA directly from WD, I suspect WD pre-registered the replacement too you. WD is a reputable maker and I cannot imagine them sending you a refurbished drive, unless it, or the documentation clearly stated that. And with 100s of millions of drives off WD assembly lines, the odds you received the same drive you sent in previously on an RMA seems extremely remote.
No company likes to keep product in inventory on the shelf. That costs money just sitting there doing nothing. So returns are tested and either (1) certified good and returned for sale, or (2) found bad and destroyed. It is likely any drives you returned were sent back out almost immediately (thus to someone else) or destroyed.
And since serial...