Hi,
You can check the OTF data in the SP02, by selecting "Goto -> Display requests -> Settings", and then select "Raw". If you display now the spool request you will see the OTF data. Here on the first page you should see the command for duplex, and on the second page it should be missing.
There is a second method to check the final print data without print it on a printer, but you need then to be able to analyse the print data(e.g. PCL or Postscript or SAPWIN print data).
You need to create a file printer on your PC as in note #576973 described, then create an output device with access method G, print the spool request over the newly created output device, and select on the Windows print popup the FILE printer. You will get a new popup for saving a file. This file contains the print data in a form which the device type supports, e.g. by a SAPWIN device type you will get a SAPWIN print data file by a PCL device type like HPLJ4 a PCL print data file.
If you analyse this on the first page you should find the command for duplex, and on the second page no duplex command.
So you can ensure whether this works correctly or not. But it is much easier to print it on a printer.
Best regards,
Norbert