detachInterrupt(0) in arduino not working ? after giving continuous ground to INT0 wakeupnow function should execute once not again and again
You're detaching in completely the wrong place. After sleep_mode() returns from sleep and before the interrupt is detached you have an entire second of time where your interrupt can still fire. And it will fire, since a LOW interrupt will keep firing while the input is LOW.

sleep_mode();     // here the device is actually put to sleep...!!
  // THE PROGRAM CONTINUES FROM HERE AFTER WAKING UP    
                  delay(500);  
                  Serial.println("Rajat2");  
                  delay(500);
detachInterrupt(0);   


Instead you should detach the interrupt within the interrupt routine itself:

void wakeUpNow() 
{  
  // execute code here after wake-up before returning to the loop() function  
  // timers and code using timers (serial.print and more...) will not work here.  
  // we don't really need to execute any special functions here, since we  
  // just want the thing to wake up  

    detachInterrupt(0);   

  int count=10;
  while(count!=0)
  {
    delay(1000);
    count--;
    Serial.println(count);  
    delay(1000);   
  }
}  


Also, all those delays and Serial prints in an interrupt are seriously problematic. An interrupt should never delay() and should never Serial.println().