1, 2, 1, 2 is not a set, so I don't know what you're referring to. Do you mean {1, 2} and {1, 2, 1, 2}? Those are sets, and they are the same set because they contain the same elements. They would also be subsets of each other.

{1} is a subset of {1, 2}. 1 is a member of {1, 2}.