So there are several reasons why this is:
C-3PO never introduced himself as C-3PO, and there were MANY protocol droids in the galaxy.
The last time Owen saw him, he was gray not gold.
Owen doesn’t have the connection to C-3PO that us Star Wars fans have. He just viewed C-3PO as a tool and nothing else. So even if he did find out that he once owned C-3PO he probably wouldn’t have cared.
Like imagine if you find a toaster that you once owned 20 years ago, except it’s gold and not grey like it used to be. That’s what it would be like pretty much.
So yeah, I know that not too many people wonder this, but if anyone was, I hope I helped.👍