In most cases, the IRS will automatically refund the extra payment as long as both payments were clearly marked for the same tax year and the taxpayer does not owe any additional funds for other years.
It may take a bit of time, especially under current conditions, to see the refund.
You can also write to the IRS - but at this time I do not recommend this as their backlog of mail is reported to exceed ten million pieces at the moment.
If possible, you may wish to see if your bank can cancel one of the payments in the mean time.
