title: find indices of duplicate floats in a list
body: <p>I have an input of a very large list of floating point numbers, a sample is given</p>
  
  <pre><code>[1.2, 2.4, 3.1, 4.0, 5.6, 6.5, 1.2, 3.1, 8.1, 23.6, 29.3]
  </code></pre>
  
  <p>I want to find all the duplicates and their index i.e. location in the list. The duplicates will only occur as a pair; never more than twice.</p>
  
  <p>The output should be like </p>
  
  <pre><code>1.2  1 7
  3.1  3 8
  </code></pre>
  
  <p>so there are just two entries 1.2 and 3.1 which occur as duplicates, and their positions are 1, 7 and 3, 8 respectively.</p>
  
  <p>any suggestions with python?</p>