Samsung s duos not booting into recovery

I have flashed the recovery.tar using odin by following this tutorial http://www.droidiser.com/2012/10/how-to-root-samsung-galaxy-s-duos-install-cwm.html but after this,when I try to boot into recovery mode by pressing volume up+volume down+home+power key simultaneously,it shows samsung logo followed by cwm recovery icon but then it just goes off and boots up normally. what am I doing wrong ? please help...