Brother HL-5280DW - Manual (Page 31)

2. Printing Methods
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5 Click OK. The printer will print on both sides of the paper automatically.
For the BR-Script printer driver
1 Open the Properties dialog box in the printer driver.
2 From the General tab, click the Printing Preferences icon.
3 From the Layout tab, choose Orientation, Print on Both Sides (Duplex) and Page Order.
4 Click OK. The printer will print on both sides of the paper automatically.