title: Create tables based on unique and nonunique values in a single column
body: <p>Given a CSV with the following structure, </p>
  
  <pre><code>id, postCode, someThing, someOtherThing
  1,E3 4AX, cats, dogs
  2,E3 4AX, elephants, sheep
  3,E8 KAK, mice, rats
  4,VH3 2K2, humans, whales
  </code></pre>
  
  <p>I wish to create two tables, based on whether the value in the <code>postCode</code> column is unique or not. The values of the other columns do not matter to me, but they have to be copied to the new tables. </p>
  
  <p>My end data should look like this, with one table based on unique <code>postCode</code>s:</p>
  
  <pre><code>id, postCode, someThing, someOtherThing
  3,E8 KAK, mice, rats
  4,VH3 2K2, humans, whales
  </code></pre>
  
  <p>And another where <code>postCode</code> values are duplicated</p>
  
  <pre><code>id, postCode, someThing, someOtherThing    
  1,E3 4AX, cats, dogs
  2,E3 4AX, elephants, sheep
  </code></pre>
  
  <p>So far I can load the data but I'm not sure of the next step: </p>
  
  <pre><code>myData &lt;- read.csv("path/to/my.csv",
    header=TRUE,
    sep=",",
    stringsAsFactors=FALSE
  )
  </code></pre>
  
  <p>New to R so help appreciated.    </p>
  
  <p><strong>Data in <code>dput</code> format.</strong>  </p>
  
  <pre><code>df &lt;-
  structure(list(id = 1:4, postCode = structure(c(1L, 1L, 2L, 3L
  ), .Label = c("E3 4AX", "E8 KAK", "VH3 2K2"), class = "factor"), 
  someThing = structure(c(1L, 2L, 4L, 3L), .Label = c(" cats", 
  " elephants", " humans", " mice"), class = "factor"), 
  someOtherThing = structure(c(1L, 3L, 2L, 4L), 
  .Label = c(" dogs", " rats", " sheep", " whales               "
  ), class = "factor")), class = "data.frame", 
  row.names = c(NA, -4L))
  </code></pre>