Printing on both sides

This keeps annoying me; I don't know why it keeps coming up.

My printer has a duplexer, but sometimes Windows 7 forgets about it. HP has a help page about it, the gist of which is:

Control Panel -> Right-click on your printer -> Printer properties -> Device Settings

Set "Duplex Unit" to "Installed" and, to reduce confusion, set "Allow manual duplexing" to "Disabled"