One user reported that he could not print in duplex in Word 2011. We tested some documents and found inconsistent results. For some documents, the duplex printing was okay. However, we could not get some documents to print in duplex.
Further investigation showed that the ones we could not print in duplex were all in the old format (.doc). After we saved the documents in the new .docx format, they printed okay. Before Microsoft has an update to fix this bug, you can just convert the documents in the old format to the new format and print them.