I had to build a new printserver because we're changing our domain.
On this new printserver, I made a queue for a HP Laserjet 4015N.
As I compared the settings of the old queue and the new queue, I saw that on the new queue, duplex-printing is enabled, even though the printer can't print on both sites without human interference. So I tried to disable it, but everytime I disable it and click on Apply and close the Properties-window, it switches back to 'Installed'.
The driver we use now is HP Universal Printing PS (v6.3.0), but on the old printserver we used v5.7.0.