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I understand you want to disable two-sided printing on your HP Envy 7640 printer. Don't worry as I have a few steps to help you get through this concern,
Let’s try to reset your printing systems first. Try the steps below:
Click the Apple menu icon, and then click “System Preferences”.
Click “Printers and Scanner” / “Print and Scan” or “Print and Fax”.
Right-click the blank space in the Printers list, and then click “Reset printing system”.
Click “Reset” in the confirmation window.
Enter an administrator username and password, and then click “OK”.
Wait until the printing system is reset and no devices show in the Printers list
Click the “Add button” (+).
Find, and then select the name of your printer from the printer list, and then click “Add” to add your printer.
Click “Print Using...”, and then select the name of your printer (By default “Airprint” would be selected)