Canon LBP7780CX User Manual page 594

[Off],
[Alternative Method]
[On]
If this item is set to [On] and if the last page for the 2-sided printing is 1-sided
printing, 1-sided printing is applied to the last page. (Printing can be performed
faster than normal 2-sided printing.)
However, when performing 2-sided printing on punched paper or previously
printed paper, the orientation or printing surface of the last page may differ from
other pages. In this case, specify [Off].
If [On] is specified, 1-sided printing is also performed in the following
cases.
The page on the back side of the sheet (even-numbered page) is
blank.
The page on the back side of the sheet (even-numbered page)
cannot be printed (for example due to page size not permitting 2-
sided printing).