The USPS website mentions nothing about not scanning tax returns and specifically mentions options for proof of mailing.
Mail Your Tax Return with USPS
The IRS will not accept an unsigned tax form, so sending an unsigned duplicate will not be accepted. If you mean, should you send a signed copy, without requiring a signature from the recipient, you can do that. IRS will reject a duplicate return so it will only process the first one it receives.
You can also drop off a copy of your return at your local IRS office.
You can send your return by private delivery carrier (FedEx, UPS, etc.)
Internal Revenue Service
333 W. Pershing Road
Kansas City, MO 64108
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