Instead of throwing an exception, return false if the layer can't be added to the map.

lib/OpenLayers/Map.js

@@ -938,16 +938,18 @@ OpenLayers.Map = OpenLayers.Class({
     *
     * Parameters:
     * layer - {<OpenLayers.Layer>} 
+    *
+    * Returns:
+    * {Boolean} True if the layer has been added to the map.
     */    
     addLayer: function (layer) {
         for(var i = 0, len = this.layers.length; i < len; i++) {
             if (this.layers[i] == layer) {
-                throw new Error("You tried to add the layer: " + layer.name +
-                                " to the map, but it has already been added");
+                return false;
             }
         }
         if (this.events.triggerEvent("preaddlayer", {layer: layer}) === false) {
-            return;
+            return false;
         }
         if(this.allOverlays) {
             layer.isBaseLayer = false;
@@ -979,6 +981,8 @@ OpenLayers.Map = OpenLayers.Class({
         this.events.triggerEvent("addlayer", {layer: layer});
         layer.events.triggerEvent("added", {map: this, layer: layer});
         layer.afterAdd();
+
+        return true;
     },
 
     /**