var loginsubmit=0;
function checkLogin(form)
{	

	if ((form.D1.value)==(""))
	{
		alert("Please enter your user name");
		return false;
	}
	else{okay = true;}
	if ((form.D2.value)==(""))
	{
		alert("Please enter your password");
		return false;
	}
	else{okay = true;}

        if (loginsubmit==0)
        {
		loginsubmit++;
		return true;
	}
        else
        {
		alert("This form has already been submitted. Thanks!");
		return false;
	}
            
}