patch from #796 - warn users if they try to add a layer that has already been added

git-svn-id: http://svn.openlayers.org/trunk/openlayers@3574 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Map.js

@@ -492,6 +492,9 @@ OpenLayers.Map.prototype = {
     addLayer: function (layer) {
         for(var i=0; i < this.layers.length; i++) {
             if (this.layers[i] == layer) {
+                var msg = "You tried to add the layer: " + layer.name + 
+                          " to the map, but it has already been added";
+                OpenLayers.Console.warn(msg);
                 return false;
             }
         }