title: Merge two DataFrames by combining duplicates and concatenating nonduplicates
body: <p>I have two DataFrames:</p>
  <pre><code>df = pd.DataFrame({'A':[1,2],
               'B':[3,4]})
      A   B
  0   1   3
  1   2   4
  
  df2 = pd.DataFrame({'A':[3,2,1],
               'C':[5,6,7]})
  
  
      A   C
  0   3   5
  1   2   6
  2   1   7
  </code></pre>
  <p>and I want to merge in a way that the column 'A' add the different values between DataFrames but merge the duplicates.</p>
  <p>Desired output:</p>
  <pre><code>  A   B   C
  0 3   NaN 5 
  1 2   4   6
  2 1   3   7
  </code></pre>