Hi, thank you very much for your tutorial. It works, but I stumbled upon some difficulties, and sharing them might be helpful for some who hasn't root their Galaxy S Duos yet.
Firstly I didn't have enough space on my SD card for the back up. It looks like the whole system is being backed up, not only the stock ROM, but all the programs that are installed so far as well, so I needed about 2 GB.
Secondly when I restarted the device after the failed back up to free some space and then restarted back to clockworkmod recovery, it was the stock recovery mode, but I couldn't spot this except for the fact that the Back & Restore was missing. It looks like if the device is restarted after being once in the custom clockworkmod recovery, goes back automatically to the stock recovery. On the custom one there is a drawing of a "tom hat" in the middle of the display.
Thirdly, when I tried to flash back to the custom clockworkmod recovery, I couldn't do it, since I wasn't removing the battery at the exact moment, or unplugging the cable after removing the battery. Just to make sure that the flash was successful: 1) remove the battery as soon as the device starts vibrating; 2) unplug the USB cable from the device 3) wait a couple of seconds 4) when you put back the battery, the device should not boot it self automatically. You should either press the power button or go to custom clockworkmod recovery mode directly if you have the needed space and files for the rest of the process.
Thanks again. Cheers!