appendchild element once if element present in js

//insert new Element after some reference Element
function insertAfter(newElement, referenceElement) {
    referenceElement.parentNode.insertBefore(newElement, referenceElement.nextSibling);
}

//example usage
var newElement = document.createElement("div");
    newElement.innerHTML = "my New Div Text";
var myCurrentElement= document.getElementById("myElementID");
    insertAfter(newElement, myCurrentElement); <script>
       var button = false;
        function  myfun(){
            if ( button === false){
                var b1 = document.createElement("BUTTON");
                b1.innerHTML = "Click Me";
                document.body.appendChild(b1); 
                return button = true;  
            }
      }
    </script>
<p id="demo" onclick="myfun();"> click me to create button </p>parentElement.insertBefore(newElement, parentElement.children[2]);