title: Take first entry in repeated measures in R
body: <p>df: </p>
  
  <pre><code>ID     Cat1  Cat2    Cat3   Cat4
  A0001   358 11.25   37428   0
  A0001   279 14.6875 38605   0
  A0013   367 5.125   40152   1
  A0014   337 16.3125 38624   0
  A0020   367 8.875   37797   0
  A0020   339 9.625   39324   0
  </code></pre>
  
  <p>want an output to keep only the first unique entry by ID, i.e.</p>
  
  <pre><code>ID     Cat1  Cat2    Cat3   Cat4
  A0001   358 11.25   37428   0
  A0013   367 5.125   40152   1
  A0014   337 16.3125 38624   0
  A0020   367 8.875   37797   0
  </code></pre>
  
  <p>Appreciate any help on this.</p>