int ledpin = 8;
int switchpin = 7;
boolean ledvalue = 1;
boolean switchvalue =1;

void setup() {
  // put your setup code here, to run once:
  pinMode(switchpin, INPUT);
  pinMode(ledpin, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  if(digitalRead(switchpin)==LOW){
    delay(20);
    if(digitalRead(switchpin)==LOW){
      if(switchvalue){
        digitalWrite(ledpin,ledvalue);
        switchvalue=0;
        ledvalue=!ledvalue;
      }
    }  
  }
  else{
    switchvalue=1;
  }
}