Registered: Jul 2004
Location: On the fringe of lunacy
There are a lot of protocall and astromech droid in the galaxy. they are so comon place that the have no reason to remember them. R2D2 does not live with the Lars family. Just 3PO.
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I recognized your foul stench when I was brought onboard.
In addition to what jedijunky1138 and darktim1 said, Owen knew what type of droid 3PO was when he saw him and knew he probably had no use for a droid like him, most likely from his previous experience with a protocol droid. unbeknownst to him, this is the exact same droid with newer coverings. Plus, the day the R2 and 3PO were purchased from the Jawas, Owen didn't spend too much time with them. He had Luke taking care of them. The only time Owen saw them was when he bought them. Then, Luke left with 3PO early the next morning to find R2, before Owen woke up, so we pretty much saw all of Owen and 3PO's time together on screen. Owen and Beru really didn't have a chance to recognize similarities in the droid they once had and the one they just bought.
And when owen realized that the droids were looking for kenobi, they recognised them. Realising who the droids were, he ordered luke to have the droids' memory erased.