title: unique indexes throwing: Reindexing only valid with uniquely valued Index objects
body: <p>So I have got a list of dataframes. When trying to append them I get:</p>
  <p><strong>&quot;pandas.errors.InvalidIndexError: Reindexing only valid with uniquely valued Index objects&quot;</strong></p>
  <p>However, my indexes are unique, example:</p>
  <pre><code>    dfList[0].index
      Index(['5560654070'], dtype='object')
  
      fList[1].index
      Index(['5562221068'], dtype='object')
  </code></pre>
  <p>When append(), or concat(), both give the error above.</p>
  <pre><code>    dfList[0].append(dfList[1])
  </code></pre>
  <p>pandas.errors.InvalidIndexError: Reindexing only valid with uniquely valued Index objects</p>
  <p>I have tried reset_index() as well as ignore_index=True, but nothing seems to work.</p>