Have they recently received a notice about their 2019 return?  I have seen some 2019 returns, without refunds, that were not processed until well into this year.  Some people received EIP#3 based on 2019, even though not eligible based on 2020.  I haven't seen one, but it's possible that some people received $1,400 based on 2019, and then another $1,400 based on 2020. 

What makes you think that it has to be returned?  At least, not until IRS can cite a statute that overpayments are not allowed.  Thousands of people are receiving overpayments, because their income or dependents have changed.