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Related to see double: double vision
to see two of everything instead of one, owing to a medical disorder. When I was driving, I saw two people on the road instead of one. I'm seeing double. There's something wrong with my eyes. Mike at first thought he was seeing double when he saw Mary with her sister. He didn't know she had a twin.
See two images of one object, either as an illusion or owing to some visual aberration, especially one caused by intoxication. For example, Those twins look so much alike they make me think I'm seeing double, or One more drink and I'll be seeing double. This idiom was first recorded in 1628.