You can use a boolean flag to indicate when the trigger can destroy the rigidbody - clear it when the rigidbody is destroyed, and set it when the player left the trigger:

private var canKill = true;

function OnTriggerEnter(x:Collider){

  if (canKill && x.tag == "???"){
    Destroy(rigidbody);
    canKill = false;
  }
}


function OnTriggerExit(){

  canKill = true;
}

set the layers on the collider of this object to ignore the triggers layer so it will not destroy it right away again, fire it out and then change the layer back