Brother DCP-J140W - Manual (Page 29)

Printing
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1
k
The machine will print all the even-numbered pages on the other side of the paper.
Guidelines for printing on both sides of the paper
1
 If the paper is thin, it may wrinkle.
 If the paper is curled, flatten it and then put it back in the paper tray.
 If the paper is not feeding properly, it may be curled. Remove the paper and straighten it.