title: appended list item appearing 2 times
body: <p>The action module takes as origin 'audit' audit module, when I try this code:
  in audit.py:</p>
  
  <pre><code> class action(orm.Model):
   _inherit = "action"
   def __init__(self, pool, cr):
      """Add a new audit value"""
      return super(action, self).Origin_SELECTION.append(('audit', 'Audit'))
  </code></pre>
  
  <p>in action.py we have:</p>
  
  <pre><code>Origin_SELECTION=[('dysfunction','Dysfunction')]
  origin_act = fields.Selection(Origin_SELECTION, 'Origin')
  </code></pre>
  
  <p>in the final selection field the 'audit' choice appeared 2 times.</p>