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I suspect that either there is a paper presence sensor switch stuck in the position that it occupies when a piece of paper is present, in the area of the Duplexer, or perhaps, that there is a fault in the control circuitry that interprets signals from the sensors, and it is sending a false signal.
Try pulling the power plug on your printer while it is On.
Then leave it that way, unpowered, for 20 minutes or longer.
Then plug it in again, and try it again.
If that does not cure it, try shaking up the duplexer, to hope to dislodge a possible stuck sensor switch. (Admittedly this would not be easy - but examination of that area for such a stuck paper presence switch might be even harder to do. Try whatever can be done.)
Let me know how that goes, please.