This Is HERE a guide about how to use sd card as internal memory,,basically this mod will reverse and make your phone think that sd card is your internal memory!!
Galaxy S Duos have a big problem of small internal memory of 1.8GB and u can t install high games on it like NFS MW which is 2GB so, This Mod Will Allow You,To Make U DO THAT!

Requirments: (things you'll need before doing this mod)

You'll need to be rooted, have root explorer installed and of course a micro sd card in your tab:

- Download and unzip the attached file and put it in the root of your internal sdcard (/mnt/sdcard)

- Open root explorer and navigate to /mnt/sdcard (if its not already there) tap and hold on the file instability and pick the copy option

- Now navigate to the /etc folder tap the Mount r/w button

- Find the file tap and hold it, then select the option rename. Change the name to install_recovery.bak and then tap paste to copy over the new file.

Now look just under the file, you'll see the date and permissions, it should say something like rwxrwxrwx if it does not you'll need to tap and hold the file tap permissions and check all three boxes for read and execute.

Once you've done the above steps reboot and wait about a minute after your tab comes up, goto settings and look at your storage space to see if the internal space size has changed.
To reverse just delete the file, rename the backed up one and reboot. If you decide to keep this mod, you'll want to copy the contents of /mnt/external_sd to /mnt/sdcard so you dont loose any of your app data. Also, you'll no longer need GL 2 SD or any other mod to fetch data from your micro sdcard.

Note: if you have problems deleting or writing to /mnt/external_sd after doing this mod, in root explorer goto /mnt press Mount R/W tap and hold on external_sd and choose permissions. Make sure that all boxes for read and write are checked.
You can use this in conjunction with ICS, its been tested and works great (actually improved my bench scores in antutu benchmark). You can merge the system r/w mod with this one to keep your system read /writable (in ICS) just add the system r/w commands to the end of this file.
Downloads -