This root is for Samsung Grand Duos users, with love.
It's essentially the same way rooting other Samsung phones: Odin, SuperSU, TWRP.
The SuperSU version I will be using is, surprisingly, v1.25 (credit to Suriya.xSe)
Please backup your files before rooting, it's always a must.
Download & install bootloader
Download Samsung Odin and SuperSU v1.25 (actually later version may also work)
Turn on and enter “developer mode”, and turn on “USB debugging mode” & “OEM unlock mode”.
Install Samsung bootloader: Install Kies on PC & restart your computer, then connect your phone with PC. It will automatically run the software.
Install TWRP Recovery
On PC, run as administer and install TWRP
Now turn on your Samsung Grand Duos
Press & hold “Power + Volume + Home” together until screen turns off & enters to a blue screen with “warning”
Now, press volume up to continue, it will show that a file is downloading to your cellphone.
Connect your phone to PC via USB cable
If successful, a notification box will show that COM is properly booted. If not, please check your phone is correctly connected.
On PC side, click Start on Odin
After a few seconds, a green “PASS” will appear on your computer screen
Now exit Odin, unplug your cellphone and start your cellphone
Turn off your device
Press “Power+Volume+Home” button together, until your phone is booted into recovery mode (You should see a recovery logo)
Optional: to make your future experience better, tick on “Never show this screen during boot again”
Slide the bar in the bottom of the screen to allow changes
Now click install &view SuperSU.zip file, or if you can locate your SuperSU.zip file. Simply click on it will do
Now wait and see if it is successfully flashed, a success note will pop out if it does
Now reboot your device, and your grand duos is ready to roll