title: Remove all duplicates except last instance
body: <p>So I have a dataset in R with the following layout as an example:</p>
  
  <pre><code>ID Date Tally
  1 2/1/2011 1
  2 2/1/2011 2
  3 2/1/2011 3
  1 2/1/2011 4
  2 2/1/2011 5
  1 2/1/2011 6
  3 2/1/2011 7
  4 2/1/2011 8
  2 2/1/2011 9
  </code></pre>
  
  <p>I want to remove all instances except the LAST instance of the post id. Right now everything I can find online, and functions I am using is removing everything except the FIRST instance.</p>
  
  <p>So my new data frame would look like:</p>
  
  <pre><code>ID Date Tally
  1 2/1/2011 6
  3 2/1/2011 7
  4 2/1/2011 8
  2 2/1/2011 9
  </code></pre>
  
  <p>How do I do this? Right now I am only able to keep the first instance. I want it to do the opposite? Any help?</p>