title: flatten list of lists in (key, List[List]) PCollection, retaining the key
body: <p>I have a PCollection of (key, value) pairs where the value is itself a list</p>
  <pre><code>{'apple': ['1','2'],
   'watermelon': ['4','5']
   'apple': ['9','10']
   }
  </code></pre>
  <p>I would like to obtain another collection with the same key, but where the list is a <code>GroupByKey</code> on the original collection, but where the resulting value is a flattened.</p>
  <pre><code>{'apple': ['1','2','9','10'],
   'watermelon': ['4','5']
   }
  </code></pre>
  <p>instead of</p>
  <pre><code>{'apple': [['1','2], ['9','10']],
   'watermelon': [['4','5']]
   }
  </code></pre>
  <p>I have tried a few things but it's not obvious to me how to go about it.</p>
  <p>I guess one can always write a <code>ParDo</code> function that flattens the list on a per-element basis but it feels there must be an easier solution.</p>