title: Removing duplicate rows based on column while keeping the highest value of the next column
body: <p>I'd like to remove the duplicates from column x1 and x2 while keeping the higher value from x3. </p>
  
  <p>DF:</p>
  
  <pre><code>x1  x2  x3
   1   1   1
   1   1   2
   1   1   3
   2   2   2
   2   2   5   
  </code></pre>
  
  <p>Expected result: </p>
  
  <pre><code>x1  x2  x3 
   1   1   3
   2   2   5
  </code></pre>
  
  <p>I've gotten as far as df[!duplicated(df[,c(1,2)]),] but it's displaying the lowest value of x3. I'd like to get the highest x3 value. </p>
  
  <p>Thanks ahead of time. </p>