title: How to filter for unique combination of columns from an R dataframe
body: <p>I am trying to filter out the duplicates of a subset of columns from a dataframe in R.</p>
  
  <p>I am interested in filtering unique combinations of session, first, and last. The following is what my data looks like</p>
  
  <pre><code>                        session       first     last            city
  1  9cf571c8faa67cad2aa9ff41f3a26e38     cat   biddix          fresno
  2  e30f853d4e54604fd62858badb68113a   caleb     amos                
  3  2ad41134cc285bcc06892fd68a471cd7  daniel  folkers                
  4  2ad41134cc285bcc06892fd68a471cd7  daniel  folkers                
  5  63a5e839510a647c1ff3b8aed684c2a5 charles   pierce           flint
  6  691df47f2df12f14f000f9a17d1cc40e       j    franz prescott+valley
  7  691df47f2df12f14f000f9a17d1cc40e       j    franz prescott+valley
  8  b3a1476aa37ae4b799495256324a8d3d  carrie mascorro            brea
  9  bd9f1404b313415e7e7b8769376d2705    fred  morales       las+vegas
  10 b50a610292803dc302f24ae507ea853a  aurora      lee                
  11 fb74940e6feb0dc61a1b4d09fcbbcb37  andrew    price       yorkville
  </code></pre>