Update build.

dist/leaflet.markercluster-src.js

@@ -21,7 +21,9 @@ L.MarkerClusterGroup = L.FeatureGroup.extend({
 		zoomToBoundsOnClick: true,
 		singleMarkerMode: false,
 
-		disableClusteringAtZoom: null
+		disableClusteringAtZoom: null,
+
+		skipDuplicateAddTesting: false
 	},
 
 	initialize: function (options) {
@@ -40,8 +42,7 @@ L.MarkerClusterGroup = L.FeatureGroup.extend({
 
 	addLayer: function (layer) {
 
-		if (layer instanceof L.LayerGroup)
-		{
+		if (layer instanceof L.LayerGroup) {
 			for (var i in layer._layers) {
 				if (layer._layers.hasOwnProperty(i)) {
 					this.addLayer(layer._layers[i]);
@@ -66,6 +67,10 @@ L.MarkerClusterGroup = L.FeatureGroup.extend({
 			return this;
 		}
 
+		if (!this.options.skipDuplicateAddTesting && this.hasLayer(layer)) {
+			return this;
+		}
+
 		//If we have already clustered we'll need to add this one to a cluster
 
 		if (this._unspiderfy) {
@@ -117,6 +122,20 @@ L.MarkerClusterGroup = L.FeatureGroup.extend({
 		return this;
 	},
 
+	hasLayer: function (layer) {
+		var res = false;
+
+		this._topClusterLevel._recursively(new L.LatLngBounds([layer.getLatLng()]), 0, this._map.getMaxZoom() + 1,
+			function (cluster) {
+				for (var i = cluster._markers.length - 1; i >= 0 && !res; i--) {
+					if (cluster._markers[i] === layer) {
+						res = true;
+					}
+				}
+			}, null);
+		return res;
+	},
+
 	//Overrides FeatureGroup.onAdd
 	onAdd: function (map) {
 		L.FeatureGroup.prototype.onAdd.call(this, map);