title: Remove multiple columns from data.table
body: <p>What's the correct way to remove multiple columns from a data.table?  I'm currently using the code below, but was getting unexpected behavior when I accidentally repeated one of the column names.  I wasn't sure if this was a bug, or if I shouldn't be removing columns this way.</p>
  
  <pre><code>library(data.table)
  DT &lt;- data.table(x = letters, y = letters, z = letters)
  DT[ ,c("x","y") := NULL]
  names(DT)
  [1] "z"
  </code></pre>
  
  <p>The above works fine, but</p>
  
  <pre><code>DT &lt;- data.table(x = letters, y = letters, z = letters)
  DT[ ,c("x","x") := NULL]
  names(DT)
  [1] "z"
  </code></pre>