Question: Why didn't Peter retaliate against Bob for punching him and letting the town know what a jerk he is?
Question: How is it that everybody except Bob mistook Peter for Luke? Luke was well-known by the whole town so it seems kind of strange that they would mistake a complete stranger with amnesia for someone they've actually known for a long time.
Answer: The father showed a picture of Luke. Peter and Luke were identical twins.
I'm hoping that this answer is implying that Peter and Luke are spitting images, and not actual identical twins, as there is no relation between the two men. Adele had her theories that Peter was not Luke, and Emmett stated that he knew at the reunion party. The town is fooled for a couple of reasons, mainly because he looks just like him. But also because they wanted to believe it was him. The town had suffered a lot of heartbreak after the war, and this was some good news, while it lasted.
Question: Several people from Lawson eventually realise the truth that Peter isn't really Luke. Bob knew from the beginning. Adele suspected that Peter wasn't really Luke and Emmet knew because Luke loved classical music. Since they knew that Peter wasn't really Luke, why didn't they tell people the truth? For that matter, why didn't they tell Peter the truth? Why did they let him continue thinking he is Luke?
Answer: Emmett was the only person that knew for sure. And he answers your question in the movie. "The town needed you to be Luke." He knew, but he did not want to rain on everyone's joy. Adele also eludes to the fact that she was pretty much fooling herself because she wanted to believe it. Bob also kept quiet for a similar reason to Emmett. He did not want to take away from the town's joy. Especially after so much loss and heartbreak everyone had been through. He was willing to put his dislike for Peter to the side for the time being.
Answer: The towns people felt sorry for Luke, He was widowed and had only one child. You 've seen the images on the news today, of entire families breaking down and crying when the sons, brothers, fathers and husbands returning from active duty. They wanted Luke to have some happiness in his life after so much sorrow.
Answer: Two possible reasons. 1. Hitting him back and telling everybody about Bob being a complete jerk would make him no better then Bob. 2. Bob actually knew the real Luke and if Peter had hit him, Bob could retaliate by telling everybody the truth.