HP LaserJet P2015dn shows only manual duplex under Windows 7, but auto duplex unit is installed

I set up a Computer with Windows 7 and a HP LaserJet P2015dn connected to it. All Drivers were installed by Windows during Setup.
But when printing, the Options for Duplex in Printer Settings only showed "Manual Duplex", but my Printer has an Auto Duplex Unit installed.

How-i-fixed it:

In found the solution here:

http://h30434.www3.hp.com/t5/Printer-All-in-One-Software-Drivers-e-g-Windows-8/Missing-auto-duplex-option-in-Win-7-for-LaserJet-P2015dn/td-p/675523

1. Click on the Start button.

2. Click on Devices/Printers.

3. Once the windows screen has popped up....scroll down to your Printers/Faxes section.

4. Right click on the P2015 printer icon.

5. In the drop down menu, select "Printer Properties"

6. Once the Printer Properties dialog box has opened, click on the device settings tab.

7. Scroll down to the Duplex Unit line and make sure this is set to installed. If not, use the pull down tab and change to installed.  (see below pic.).