diff --git a/src/micro/behaviors.html b/src/micro/behaviors.html
index 7c909de1..bd841d85 100644
--- a/src/micro/behaviors.html
+++ b/src/micro/behaviors.html
@@ -65,6 +65,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     },
 
     _desugarSomeBehaviors: function(behaviors) {
+      var behaviorSet = [];
       // iteration 1
       behaviors = this._flattenBehaviorsList(behaviors);
       // iteration 2
@@ -72,9 +73,13 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       // `_mixinBehavior` has _first property wins_ behavior, this is done
       // to optimize # of calls to `_copyOwnProperty`
       for (var i=behaviors.length-1; i>=0; i--) {
-        this._mixinBehavior(behaviors[i]);
+        var b = behaviors[i];
+        if (behaviorSet.indexOf(b) === -1) {
+          this._mixinBehavior(b);
+          behaviorSet.unshift(b);
+        }
       }
-      return behaviors;
+      return behaviorSet;
     },
 
     _flattenBehaviorsList: function(behaviors) {
@@ -141,7 +146,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 
   });
 
-  // special properties on behaviors are not mixed in and are instead 
+  // special properties on behaviors are not mixed in and are instead
   // either processed specially (e.g. listeners, properties) or available
   // for calling via doBehavior (e.g. created, ready)
   Polymer.Base._behaviorProperties = {
diff --git a/test/unit/behaviors.html b/test/unit/behaviors.html
index 09e02221..266bf059 100644
--- a/test/unit/behaviors.html
+++ b/test/unit/behaviors.html
@@ -133,21 +133,33 @@ suite('multi-behaviors element', function() {
       assert.match(message, /behavior is null/);
       warned = true;
     };
-
     Polymer({
-
+      is: 'behavior-null',
       behaviors: [
         null
-      ],
-
-      is: 'behavior-null'
-
+      ]
     });
-
     assert.equal(warned, true, 'Null behaviour should generate warning');
     Polymer.Base._warn = oldWarn;
   });
 
+  test('behavior array is unique', function() {
+    Polymer({
+      is: 'behavior-unique',
+      behaviors: [Polymer.BehaviorA, Polymer.BehaviorA]
+    });
+    assert.equal(document.createElement('behavior-unique').behaviors.length, 1);
+  });
+
+  test('duplicate behaviors keep last behavior', function() {
+    Polymer({
+      is: 'behavior-unique-last-behavior',
+      behaviors: [[Polymer.BehaviorA, Polymer.BehaviorB], Polymer.BehaviorA]
+    });
+    var behaviors = document.createElement('behavior-unique-last-behavior').behaviors;
+    assert.deepEqual(behaviors, [Polymer.BehaviorB, Polymer.BehaviorA]);
+  });
+
 });