Utilize your financial knowledge, give your answer or opinion to the input question or subject . Answer format is not limited.

Using 2 different social security numbers
Social security number should only be needed for things that involve tax withholding or tax payment. Your bank or investment broker, and your employer, need it so they can report your earnings. You need it when filing tax forms. Other than those, nobody should really be asking you for it.  The gym had absolutely no good reason to ask and won't have done anything with the number. I think we can ignore that one. The store cards are a bigger problem. Depending on exactly what was done with the data, you may have been messing up the credit record of whoever legitimately had that number... and if so you might be liable on fraud charges if they or the store figure out what happened and come after you. But that's unrelated to the fact that you have a legitimate SSN now. Basically, you really don't want to open this can of worms. And I hope you're posting from a disposable user ID and not using your real name... (As I noted in a comment, the other choice would be to contact the authorities (I'm not actually sure which bureau/department would be best), say "I was young, foolish, and confused by America's process... do I need to do anything to correct this?", and see what happens... but it might be wise to get a lawyer's advice on whether that's a good idea, a bad idea, or simply unnecessary.)