Even though all characters seem to tumble over each other during the whole of SW and keep running into each other, we never ever see another Protocol Droid identical to 3PO, nor an astromech identical to R2.
We've seen a chrome, a white and a red 3PO, but the only golden one is 3PO. Also, his voice is very unique compared to those other ones.
We've also seen many different astromecyhs, but none were identical to R2 in every bit.
Vader, even glancing over to 3PO, or especially hearing his voice, should have a moment of recognition, not neccesarily that that is HIS 3PO, but at least something.
But, it could very well be that, like his true name, that droid he made as a kid has no meaning for him anymore.