Throw a byte-saving AssertionError

doc/errors/index.md

@@ -220,3 +220,7 @@ The `{-y}` placeholder requires a tile grid with extent.
 ### 57
 
 At least 2 conditions are required.
+
+### 58
+
+Duplicate item added to a unique collection.  For example, it may be that you tried to add the same layer to a map twice.  Check for calls to `map.addLayer()` or other places where the map's layer collection is modified.

src/ol/collection.js

@@ -6,6 +6,7 @@
 goog.provide('ol.Collection');
 
 goog.require('ol');
+goog.require('ol.AssertionError');
 goog.require('ol.CollectionEventType');
 goog.require('ol.Object');
 goog.require('ol.events.Event');
@@ -256,7 +257,7 @@ ol.Collection.prototype.updateLength_ = function() {
 ol.Collection.prototype.assertUnique_ = function(elem, opt_except) {
   for (var i = 0, ii = this.array_.length; i < ii; ++i) {
     if (this.array_[i] === elem && i !== opt_except) {
-      throw new Error('Duplicate item added to a unique collection');
+      throw new ol.AssertionError(58);
     }
   }
 };