title: pandas ValueError: cannot reindex from a duplicate axis when trying to do calculation based on values from another df
body: <p>I have 2 <code>df</code>s:</p>
  
  <pre><code>df2
  dec_pl    cur_key
  0         JPY
  1         HKD
  
  df1
  cur    amount
  JPY    80
  HKD    20
  USD    70
  </code></pre>
  
  <p>I like to reference <code>del_pl</code> in <code>df2</code> for 'cur' in <code>df1</code>, and calculate <code>df1.converted_amount = df1.amount * 10 ** (2 - df2.dec_pl)</code> for <code>df1</code>; i.e. <code>df1.amount</code> times the 10 to the power of <code>(2 - df2.dec_pl)</code> and if there cannot find a corresponding <code>df2.cur_key</code> from <code>df1.cur</code>, e.g. <code>USD</code>, then just use its amount;</p>
  
  <pre><code>df1 = df1.set_index('cur')
  df2 = df2.set_index('cur_key')
  df1['converted_amount'] = (df1.amount*10**(2 - df2.dec_pl)).fillna(df1['amount'], downcast='infer')
  </code></pre>
  
  <p>but i got </p>
  
  <pre><code>ValueError: cannot reindex from a duplicate axis 
  </code></pre>
  
  <p>I am wondering whats the best way to do this, so the results should look like,</p>
  
  <pre><code>df1
  cur    amount    converted_amount
  JPY    80        8000
  HKD    20        200
  USD    70        70
  </code></pre>