title: filter unique rows ignoring NA
body: <p>These are my example data:</p>
  
  <pre><code>df &lt;- data.frame("x" = c(1,1,2,3,4,NA,NA,6), "y"=c(1,1,6,7,8,9,9,10))
  
     x  y
  1  1  1
  2  1  1
  3  2  6
  4  3  7
  5  4  8
  6 NA  9
  7 NA  9
  8  6 10
  </code></pre>
  
  <p>I'd like to get rid of duplicate entries (=rows), but I want to keep entries with at least one NA. As a result, only the second row above should be deleted, while the 7th should be kept. <code>unique(df, incomparables=NA)</code> yields an error message.</p>