HP LaserJet P2015 models use “Universal” driver under Windows 7. When first installed, duplexing doesn’t work by default. This is a surprise for many who come from Windows XP environment where the dedicated drivers had P2015’s built-in duplexing availability turned on, of course, by default. In Windows 7, however, the duplexing selector for P2015 says by default: “Print on both sides (manually)”.
To enable P2012’s automatic duplexing on Windows 7 go to Devices and Printers, highlight the P2015, right click, and select Printer Properties from the context popup menu. Go to Device Settings tab and look for “Duplex Unit (for 2-Sided Printing)” option. It’s set to “Not Installed” by default. Change it to “Installed” and click on “OK”. Now when you go to Preferences when getting ready to print, the automatic (not “manually”) option is available on the Finishing tab, and automatic duplexing works.
Also if you use the excellent priPrinter utility, P2015 honors the “Double Sided” toggle on the Page Layout tab. Same goes for the “Double-sided” checkbox on FinePrint utility, and other applications that provide the option to turn automatic duplexing on or off.