title: BASH - Find duplicates in multiple files
body: <p>I have multiple files in the same directory, each file represents a user and contains IP's used to log into this account, each in a new line.</p>
  
  <p>I want to create a script that will check if the same IP occurs in multiple files and of course print duplicates.</p>
  
  <p>I've tried using awk but with no luck, any help appreciated!</p>