When using !Ljduplex you first print a single copy of your work to a file. (This involves a very simple change to the printer from the icon bar). This file is then used by !Ljduplex to print as many copies as you wish in full duplex.

!LjDuplex will only work with B/W printers which use the Hewlett-Packard PCL printer language. Also if you have a Postscript printer there are far simpler ways of duplexing.