title: grep or awk only lines that do not have the same line immediately after
body: <p>I have a file as as follows:</p>
  
  <pre><code>abc
  abc
  123
  xyz
  foo
  foo
  bar
  </code></pre>
  
  <p>What I'm trying to do is determine which rows in the file are unique, perhaps using grep or awk.  So in this case my command should output</p>
  
  <pre><code>123
  xzy
  bar
  </code></pre>
  
  <p>...and ignore lines that contain a string that occurs again.  The file has already been sorted.  Thanks in advance.</p>