Hi, I've the same problem as reported here in 2008: http://lists.alioth.debian.org/pipermail/sane-devel/2008-July/022451.html
So when I do scanadf -d hp5590 --source "ADF Duplex" I get one image file with the front site on the top and the reverse site on the bottom flipped by 180?. I searched the mailing list but could not find any solution. Please CC me in replies. Regards, Thomas Koch, http://www.koch.ro
