In an older movie Man with my face, Barry Nelson plays a man who finds there is someone else who is an exact replica to him they look like twins.

He is being set up by his wife and a friend as well as he lookalike to rob his company...when he comes home the look a like is there, and his wife pretends she doesn't know him. When he and the duplicate are in the same scene together, he looks right at him. so someone has to be standing in so it looks like he's talking right to him. However,when he contacts the police and the policeman comes to the house the duplicate is sitting, when the policeman enters the room he is looking directly above him. he should have been looking down because he was seated. Maybe a stand in wasn't used. Later on at a friend's house they are in front of a window, when you look at the window you can see someone walk by. Yet later when they look out of the window they are on a second floor. Man couldn't have been walking on air.