have you checked after setting CAP_DUPLEXENABLED if it was really set correctly? I’m not very good in reading code, but I can’t find where you set the value of your cap-structure for CAP_DUPLEXENABLED to TRUE.
why are you checking CAP_DUPLEX after setting CAP_DUPLEXENABLED?
as far as I understand gabe’s post, you have to check CAP_DUPLEX first in order to find out if duplex scanning is supported and then you can set CAP_DUPLEXENABLED to True