title: Distinct Element across dstream
body: <p>I am working on window dstreams wherein each dstream contains 3 rdd with following keys:</p>
  
  <pre><code>a,b,c
  b,c,d
  c,d,e
  d,e,f
  </code></pre>
  
  <p>I want to get only unique keys across all dstream </p>
  
  <pre><code>a,b,c,d,e,f
  </code></pre>
  
  <p>How to do it in spark streaming? </p>