Google Maps Multiple markers with the exact same location Not working

You need to :

1. create the marker clusterer first.

2. add the markers to the MarkerClusterer (and format your code so it is easier to read…).

   function createMarker(latlng,text) {
     var marker = new google.maps.Marker({
       position: latlng,
       map: map
     });
   
     ///get array of markers currently in cluster
     var allMarkers = mc.getMarkers();
   
     //check to see if any of the existing markers match the latlng of the new marker
     if (allMarkers.length != 0) {
       for (i=0; i < allMarkers.length; i++) {
         var existingMarker = allMarkers[i];
         var pos = existingMarker.getPosition();
   
         if (latlng.equals(pos)) {
           text = text + " & " + content[i];
         }                   
       }
     }
   
     google.maps.event.addListener(marker, 'click', function() {
       infowindow.close();
       infowindow.setContent(text);
       infowindow.open(map,marker);
     });
     mc.addMarker(marker);
     return marker;
   }
   

working example