title: How do I find duplicate indices in a DataFrame?
body: <p>I have a pandas DataFrame with a multi-level index ("instance" and "index"). I want to find all the first-level ("instance") index values which are non-unique and to print out those values.</p>
  
  <p>My frame looks like this:</p>
  
  <pre><code>                     A
  instance  index      
        a       1      10
                2      12
                3      4
        b       1      12
                2      5
                3      2 
        b       1      12
                2      5
                3      2
  </code></pre>
  
  <p>I want to find "b" as the duplicate 0-level index and print its value ("b") out.</p>