Galaxy S Duos must be ROOTED!!
Just follow the tutorial, its very easy.
1. Download Root External 2 Internal SD here. (click skip ad after 5s)
2. Run app, grant root access and click settings on top right corner.
3. Check Default SGS3 Roms.
4. Click original internal mounting point. (Make sure it's /mnt/sdcard)
5. Click original external mounting point. (Make sure its /mnt/extSdcard)
6. Click ext. sdcard device access. (Change to /dev/block/vold/179:33)
7. Check Use per default: Check to use custom attributes
8. Check send rescan
9. Click to left corner to go back splash page.
10. Click External>Internal
11. Check At boot
12. Click At boot if init.d enable!
If there are any problems, please posta comment!