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;
     },
 
     /**

tests/Map.html

@@ -726,7 +726,7 @@
     }
 
     function test_Map_double_addLayer(t) {
-        t.plan( 1 );    
+        t.plan(2);
 
         map = new OpenLayers.Map($('map'));
         layer = new OpenLayers.Layer.WMS('Test Layer', 
@@ -734,12 +734,10 @@
                     {map: '/mapdata/vmap_wms.map', layers: 'basic', format: 'image/jpeg'} 
                     );
 
-        map.addLayers([layer]);
-        try {
-            map.addLayers([layer]);
-        } catch(e) {
-            t.ok(true, "Map does not allow double adding of layers." );
-        }
+        var added = map.addLayer(layer);
+        t.ok(added === true, "Map.addLayer returns true if the layer is added to the map.");
+        var added = map.addLayer(layer);
+        t.ok(added === false, "Map.addLayer returns false if the layer is already present.");
     
         map.destroy();
     }