title: Subset dataframe based on field
body: <pre><code>mukey   cokey     hzdept_r  hzdepb_r
  422927  11090397    0        20
  422927  11090397    20       71
  422927  11090397    71       152
  422927  11090398    0        18
  422927  11090398    18       117
  422927  11090398    117      152
  </code></pre>
  
  <p>I would like to subset the dataframe above, such that only the FIRST SET of cokey's (in this case 11090397) are selected. Of course, since this is a sample dataset, the solution needs to scale to larger versions of such a  dataframe.</p>
  
  <p>In this case, the resulting dataset should be:</p>
  
  <pre><code>mukey   cokey     hzdept_r  hzdepb_r
  422927  11090397    0        20
  422927  11090397    20       71
  422927  11090397    71       152
  </code></pre>
  
  <p>I have tried using groupby, but not sure how to select only the first cokey value from there.</p>