Printer Properties will not allow Duplex printing.
Can be overcome by using File-Print-Properties-Device and setting Duplexer to Installed but it has to be re-set every time.
Whiteboard: NeedsDuplexPrinter
The "File - Print - Properties - Device" dialog takes its defaults from printing system (which should be CUPS in the case of Linux), and does not store its own settings. If duplex printing is otherwise working, and you want it to be the global default, the right place to configure that is in CUPS rather than LibreOffice. Per-document printer settings can be set using "File - Printer Settings...". Setting RESOLVED - NOTABUG