What do you mean by duplex mode?
Are you referring to a copy machine. Duplex copies are printed
on both sides of the paper. If copying from originals, normally the
following options are available.
1 -> 1, single sided copies from single sided originals.
1 -> 2, double sided copies from single sided originals. so 4
sheets -> 2 double sided sheets.
2 -> 2, double sided originals make double sided copies.
2 -> 1, double sided originals make single sided copies. So 2
double sided originals make 4 sheets single sided copies.