Force duplex printing in Mac Snow leopard

1. Add the duplex printer first

2. edit /private/etc/cups/ppd/hp8150.ppd

3. Search “Duplex None” and replace the fragment from

*Duplex None/Off (1-Sided): ”
<</Duplex false>> setpagedevice”

To

*Duplex None/Off (1-Sided): ”
<</Duplex true /Tumble false>> setpagedevice”