What if there’s another “earth” in exactly the same orbit as ours, exactly the same size, exactly the same speed, and exactly 180 degrees of circumference away? We would never know it because it would always be behind the sun. (Perhaps there’d be gravitational impact on Mars or Venus, but maybe not)
Movie, Journey to the Far Side of the Sun, was based upon that premise.
People there exact copy of us all, except backwards alphabet, left handed, etc...................