title: Regex to match the nth (n > 1) occurrence of the same word
body: <p>I would like a regular expression that can match the nth (n > 1) occurrence of the same word in a text. I need this to detect repeating words in a text.</p>
  
  <p>Example: <em>"one two three <strong>one</strong> ten nine six <strong>one</strong> five <strong>ten</strong>"</em> 
  ( {'one','one','ten'} should be detected).</p>
  
  <p>I found several examples on how to detect repeating occurrences of a given pattern. This question is different because we don't know in advance what word(s) would be repeating. Is this even possible with regex?</p>