Not the brightest kyber crystal in the light saber.

Owen didn’t pay for C3PO twice.

Anakin built 3PO for his mom Shmi, who was a slave to Watto. Cliegg (not Owen) bought Shmi out of slavery. C3PO was already Shmi’s.

Ok, but Owen grew up with C3PO. Threepio was his mother's protocol droid, and he was around long enough to meet Beru as well.

Maybe he didn't recognize the new plating, but he should have recognized the name. He would have talked to it almost every day for his entire life.
- Do you specifically remember your old smartphone model LG KU2100? Could you identify in a stack of other smartphones? You used to use it every single day.
  
  Droids in Star Wars are appliances. Nothing memorable.
- Tatooine is clearly an acient-Rome like place where slaves own property, so only Anakin would have brought C3PO, as parts.
  
  And those 4 letter names couldn't be very unique...
- We don’t know what Owen’s relationship with Shmi was. Maybe he left home before Cliegg remarried, and was only there when we see him in episode 2 because he came when his dad contacted him in a panic after discovering Tuscan raiders had taken step-mom captive.
- In episode 3, when Senator Organa sent the droids to have their memories erased, they also used the force to make everyone (including Vader) forget about them.
Well, technically Cliegg bought 3PO when he bought Shmi. So in a roundabout way the funds of Owen's family went twice to acquiring 3PO.

Three times if you factor in the costs of however Anakin acquired the parts.

And then Owen's moody half-brother and his rich girlfriend just stole the droid.
- I disagree with your interpretation that Cliegg “technically” bought 3PO.
  
  We don’t know the legal rights of slaves in that universe, but presuming this is like slaves in ancient Roman times and they can own property, then 3PO was Shmi’s from the beginning. Buying a slave wouldn’t automatically give you possession of the slave’s property, therefore it was always Shmi’s droid and Cliegg never owned it.
  
  It’s possible that in this universe, slaves can’t own property and Cliegg bought 3PO from Watto in addition to Shmi. But I think it unlikely that Anakin would have built 3PO “to help his mom” if Watto automatically owned it. If Watto owned 3PO he would’ve sold it long before Cliegg bought Shmi.
  
  We also don’t know that Anakin stole 3PO. If it belonged to his mom, he could have simply inherited it when she died.
  
  One could also make the argument that 3PO was always Anakin’s property, and he just left it behind when he left Tattooine.