title: Python Pandas: Keeping only dataframe rows containing first occurrence of an item
body: <p>I have this:</p>
  
  <pre><code>    Date value
  0   1975     a
  21  1975     b
  1   1976     b
  22  1976     c
  3   1977     a
  2   1977     b
  4   1978     c
  25  1978     d
  5   1979     e
  26  1979     f
  6   1980     a
  27  1980     f
  </code></pre>
  
  <p>I am having trouble finding a way to keep only the lines containing the first occurrence of a 'value'. I want to drop duplicate 'values', keeping the row with the lowest 'Date'.The end result should be:</p>
  
  <pre><code>    Date value
  0   1975     a
  21  1975     b
  22  1976     c
  25  1978     d
  5   1979     e
  26  1979     f
  </code></pre>