title: Add dictionary values where keys match
body: <p>I have a list of dictionaries like the following:</p>
  
  <pre><code>a=[{'a':1},{'b':2},{'c':3},{'a':-2},{'b':4}]
  </code></pre>
  
  <p>I need to loop through the list and add the values where the keys match. The result should look like this:</p>
  
  <pre><code>a=[{'a':-1},{'b':6},{'c':3}]
  </code></pre>
  
  <p>Help doing this is appreciated.</p>