I don't own a duplex scanner, and I haven't got round to munging the
options so that they look reasonable for people with duplex scanners.

The double-sided option on the first tab is aimed at people like me
with simplex scanners who want to scan double-sided documents.

Those with duplex scanners should select single-sided paper on the
first tab, and duplex scanning on the appropriate option on a later
tab (which one varies, depending on the model of scanner).