Well, there's two possible issues, and your question is not entirely clear. If you paid the correct tax as shown on your return, then it is too late to amend, even if the return was wrong.
However, if you paid the same bill twice, that should not have happened. The IRS should have applied your second payment toward the next year, or refunded it to you. But, it might still be too late to prove the case and get the money back. Start by getting your tax return and account transcripts and your payment history from 2018 forward. See if the tax paid matches the returns that were filed, and see if all your payments are properly accounted for.
https://www.irs.gov/individuals/get-transcript
