title: dask df.col.unique() vs df.col.drop_duplicates()
body: <p>In dask what is the difference between</p>
  
  <pre><code>df.col.unique()
  </code></pre>
  
  <p>and</p>
  
  <pre><code>df.col.drop_duplicates()
  </code></pre>
  
  <p>Both return a series containing the unique elements of <code>df.col</code>.
  There is a difference in the index, <code>unique</code> result is indexed by 1..N while <code>drop_duplicates</code> indexed by an arbitrary looking sequence of numbers.</p>
  
  <p>What is the significance of the index returned by <code>drop_duplicates</code>?</p>
  
  <p>Is there any reason to use one over the other if the index is not important?</p>