title: Find out if column in R table includes duplicate values?
body: <p>I've got a lovely dataframe, my very first, and I'm starting to get the hang of R. One thing I haven't been able to find is a test for duplicate values. I have one column that I'm pretty sure is all unique values, but I don't know that.</p>
  
  <p>Is there a way I can ask? For simplicity, let's pretend this is my data:</p>
  
  <pre><code>  var1 var2 var3
  1    1    A    1
  2    2    B    3
  3    3    C   NA
  4    4    D   NA
  5    5    E    4
  </code></pre>
  
  <p>and I want to know whether <code>var1</code> ever repeats.</p>