title: BASH - Tell if duplicate lines exist (y/n)
body: <p>I am writing a script to manipulate a text file.</p>
  
  <p>First thing I want to do is check if duplicate entries exist and if so, ask the user whether we wants to keep or remove them.</p>
  
  <p>I know how to display duplicate lines if they exist, but what I want to learn is just to get a yes/no answer to the question "Do duplicates exist?"</p>
  
  <p>It seems <code>uniq</code> will return <code>0</code> either if duplicates were found or not as long as the command completed without issues.</p>
  
  <p>What is that command that I can put in an <code>if</code>-statement just to tell me if duplicate lines exist?</p>
  
  <p>My file is very simple, it is just values in single column.</p>