title: Find Duplicate/Repeated or Unique words in file spanning across multiple lines
body: <p>in Linux, I have a text file which have duplicate words like this</p>
  
  <pre><code>abc line 1
  xyz zzz
  123 456
  abc end line
  </code></pre>
  
  <p>Now I want to print only all DUPLICATE words (which is abc)
  how ?</p>