Epson AcuLaser C8600 Reference Manual page 39

All sizes except A3W:
A3W size:
5. Set the Paper Source and Paper Size settings to match the
loaded media through the printer driver or the control panel.
Note:
To print on the back side of previously printed paper, use the MP
tray. The optional Duplex Unit cannot be used for this purpose.
When you output your printouts to the face-up tray, you need to
change the Out Bin setting in the printer driver or in the Setup
Menu.
Paper Handling
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