title: Pandas Merge - How to avoid duplicating columns
body: <p>I am attempting a merge between two data frames.  Each data frame has two index levels (date, cusip).  In the columns, some columns match between the two (currency, adj date) for example.</p>
  
  <p>What is the best way to merge these by index, but to not take two copies of currency and adj date.</p>
  
  <p>Each data frame is 90 columns, so I am trying to avoid writing everything out by hand.</p>
  
  <pre><code>df:                 currency  adj_date   data_col1 ...
  date        cusip
  2012-01-01  XSDP      USD      2012-01-03   0.45
  ...
  
  df2:                currency  adj_date   data_col2 ...
  date        cusip
  2012-01-01  XSDP      USD      2012-01-03   0.45
  ...
  </code></pre>
  
  <p>If I do:</p>
  
  <pre><code>dfNew = merge(df, df2, left_index=True, right_index=True, how='outer')
  </code></pre>
  
  <p>I get </p>
  
  <pre><code>dfNew:              currency_x  adj_date_x   data_col2 ... currency_y adj_date_y
  date        cusip
  2012-01-01  XSDP      USD      2012-01-03   0.45             USD         2012-01-03
  </code></pre>
  
  <p>Thank you!
      ...</p>