Note that in 10.1.4 the suggested forum method does not work and DuplexMode does not exist in the Adobe Reader Plist http://forums.adobe.com/message/4244334
"With the 10.1.2 update, Duplex has been turned ON by default for printing. Is this the version that you are using? Here is an article with further information on Adobe Reader/Acrobat 10.1.2 sets Duplex ON by default for printing. Apparently, the following key gets generated automatically when a user tries to print on a duplex printer and then closes Reader/Acrobat 10.1.2 in the Mac preference file setting:
[user]/Library/preferences/[com.adobe.Reader.plist OR com.adobe.Acrobat.Pro.plist]
The name of the key is DuplexMode under AVGeneral and it has Number/int data type.
1 = Simplex (no duplex)
2 = Duplex with Long edge
3 = Duplex with Short edge
NOTE: 0 (or key does not exist) = Duplex with Long edge"
I tried to go back to a previous version 10.1.3 but when I edit the DuplexMode using PLISTEdit Pro it would simply remove the DuplexMode key and still not set printing to 2 sides by default.