Vegetius,
So, as far as I know (from looking at the product spec sheet) the OfficeeJet 5510 doesn't support duplexing. The sure fire way to tell if the driver thinks it supports it is to open the desktop printer browser, click on the Printer Setup button, and under the Driver tab, if the driver thinks that a duplexer is an option you'll see a checkbox next to a string similar to "HP Two-sided Printing Accessory (Duplexer). If your device does have a duplexer on it (you have printed two sided pages off of it) and the screen I just mentioned doesn't have the Duplexer option on it, then clearly we missed it do to improper documentation. If this is the case, let me know and I'll make sure that we get the driver rolled as soon as possible with the addition of the missing duplexer. If it was as simple as checking this box, well then that is a lot easier for everyone.
Just trying to help.
Andrew