<propertyName>Changed may get called twice for a single property value change

[frankiefu]

When you run the below code you will see x-bar's valueChanged() is being called twice. Debugging shows that in instance/properties.js, this._observeNames has ["value", "value"], and that causing it to register 2 observers on the same property. I believe the root cause is in declaration/properties.js, optimizePropertyMaps() simply concats base's observeNames and this observeNames, and doesn't remove duplicates.
https://github.com/Polymer/polymer/blob/master/src/declaration/properties.js#L25

<polymer-element name="x-foo" attributes="value">
  <script>
    Polymer('x-foo', {
      value: null,
      valueChanged: function() {
      }
    });
  </script>
</polymer-element>

<polymer-element name="x-bar" extends="x-foo">
  <script>
    Polymer('x-bar', {
      valueChanged: function() {
        console.log('x-bar valueChanged:', this.value);
      }
    });
  </script>
</polymer-element>

<x-bar></x-bar>

<script>
  document.addEventListener('WebComponentsReady', function() {
    document.querySelector('x-bar').value = {};
  });
</script>

output:

x-bar valueChanged: Object {}
x-bar valueChanged: Object {} 

p.s. This issue is only happening in master branch.