title: Remove duplicate values in string for all values in a column of a data frame
body: <p>I have a data frame like df below.</p>
  
  <pre><code>c1 &lt;- c(4, 3, 6, 5, 2)
  c2 &lt;- c("AAA, AAA, VAA, LLA", "CEA, CEA, HEA, CKA, NEA, TEA", "RFA, LIA, VIA, CIA, CIA, RKA", "JHD, JKE, JKE, HGH", "JII, JII, ISA, KSA")
  df &lt;- data.frame(c1, c2)
  </code></pre>
  
  <p>I want to get rid of any duplicates for the values in the second column. So for example, it should remove one of the "AAA" in the second column for the value corresponding to "4" (first row). How can this be done for all cells in the second column?</p>
  
  <p>Thanks!</p>