title: pandas: A value is trying to be set on a copy of a slice from a DataFrame
body: <p>I have a very simple command in pandas like:</p>
  
  <pre><code>volume_related_pd.loc[:,"last_record_volume"] = volume_related_pd.loc[:,"volume"]
  </code></pre>
  
  <p>I think the complexity is that i have duplicate index. It produces the warning that:</p>
  
  <blockquote>
    <p>/anaconda2/lib/python2.7/site-packages/pandas/core/indexing.py:601:
    SettingWithCopyWarning:  A value is trying to be set on a copy of a
    slice from a DataFrame. Try using .loc[row_indexer,col_indexer] =
    value instead</p>
  </blockquote>
  
  <p>What shall i do? Thanks. I already use .iloc</p>