The IRS is very behind on processing returns and may not have gotten to yours yet.  You can mail another copy, but be sure you take it to the post office and have it send so that you get a notice when it is received by the IRS.

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@Farmgirl1231 wrote:

I mailed a paper return in mid-March 2021, but the "Where's My Refund" does not show that it was received.  Do I wait until the IRS sends me a letter or should I mail a 2nd return?  Or should I do something else?

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Not a good idea.   If you mail a 2nd return and there is already a return in the IRS system then the IRS will red flag both returns for human inspection which can delay things for up to a year.