title: python list of dictionaries find duplicates based on value
body: <p>I have a list of dicts:</p>
  <pre><code>a =[{'id': 1,'desc': 'smth'},
      {'id': 2,'desc': 'smthelse'},
      {'id': 1,'desc': 'smthelse2'},
      {'id': 1,'desc': 'smthelse3'}]
  </code></pre>
  <p>I would like to go trough the list and find those dicts that have the same <code>id</code> value (e.g. <code>id=1</code>) and create a new dict:</p>
  <pre><code>b = [{'id':1, 'desc' : [smth, smthelse2,smthelse3]}, 
       {'id': 2, 'desc': 'smthelse'}]
  </code></pre>