Hi,
The printer do not support auto duplexing with US Legal paper.
You may find its specifications below, see 'Paper-handling specifications > Supported paper sizes > HP OfficeJet 8702, OfficeJet Pro 8710, 8715, 8718, and 8719 printers' section, check the Duplexer column:
https://support.hp.com/us-en/document/c04959111
Regards,
Shlomi


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