I went from Ubuntu 12 to 14 to get LibreOffice 4.2, reading here that 4.2 worked perfectly: no luck
I’ve set both file preferences and printer preferences to Long Edge (standard). Other applications print double-sided fine, but not LibreOffice.
Two ideas: (1) Change the printing type to postscript, see Change default printer language type to PostScript (Level from driver) (2) Set duplex directly in the printer properties and check both "Use only paper … from printer preferences in tab Options in printing dialog.