I've said this before and I'll say it again. Owen didn't know that this was Artoo and Threepio right off. It isn't until Luke says that he came across Leia's recorded message to Obi-wan, that Owen and Beru realized who the droids were. That's why he wanted Luke to take them to Anchorhead and have their memories wiped. As to Obi-wan, he didn't know that was Artoo right off until Luke said his real name. He then realized it was Artoo and when he saw Threepio, he recognized him as well. Not to mention that Luke said their names. And as it has been discussed, Obi-wan never owned a droid. BTW, there are a few R2 units that look the same as Artoo, in TPM. So it's not uncommon for their to be similar looking Astromech droids.