Require that map layers are unique

src/ol/layer/group.js

@@ -54,14 +54,14 @@ ol.layer.Group = function(opt_options) {
 
   if (layers) {
     if (Array.isArray(layers)) {
-      layers = new ol.Collection(layers.slice());
+      layers = new ol.Collection(layers.slice(), {unique: true});
     } else {
       ol.asserts.assert(layers instanceof ol.Collection,
           43); // Expected `layers` to be an array or an `ol.Collection`
       layers = layers;
     }
   } else {
-    layers = new ol.Collection();
+    layers = new ol.Collection(undefined, {unique: true});
   }
 
   this.setLayers(layers);

test/spec/ol/map.test.js

@@ -50,6 +50,27 @@ describe('ol.Map', function() {
 
   });
 
+  describe('#addLayer()', function() {
+    it('adds a layer to the map', function() {
+      var map = new ol.Map({});
+      var layer = new ol.layer.Tile();
+      map.addLayer(layer);
+
+      expect(map.getLayers().item(0)).to.be(layer);
+    });
+
+    it('throws if a layer is added twice', function() {
+      var map = new ol.Map({});
+      var layer = new ol.layer.Tile();
+      map.addLayer(layer);
+
+      var call = function() {
+        map.addLayer(layer);
+      };
+      expect(call).to.throwException();
+    });
+  });
+
   describe('#addInteraction()', function() {
     it('adds an interaction to the map', function() {
       var map = new ol.Map({});