title: unix text file find double entries
body: <p>I've the following file which has entries like this:</p>
  
  <pre><code>cn: 5f78ee78-54a6-41b7-bff5-a34ff4f7fc4d  
  7379800474  
  cn: f6c16630-a194-4e64-8bb9-5edab380340a  
  2193668474  
  cn: 2ed03469-eaf2-4f0a-b5b2-4e0b3fab402c  
  5051108453  
  cn: d28fc57c-0ff8-4b08-9120-6614e35da185  
  9518351168  
  cn: 3762c0ec-2a6e-4bf9-afe5-bf144e1f658d  
  4902143367  
  cn: 600bdcfe-bf0a-4735-95bc-fbe1f2d521d5  
  3239279952  
  cn: 8d7d1204-4d5b-48ab-8f2c-3ab5d7da2d3d  
  3339625694  
  3546846448  
  3339625694  
  3546846448  
  cn: e52f7b5a-9061-4168-9622-f4f0c3cd863c  
  4853840278  
  cn: e617bf31-58a9-412f-9cac-77620f203c1b  
  8075983683  
  cn: c42d6609-544b-412e-abd5-c76eb1fa7ec1  
  3938895718  
  cn: 33359ce9-e57e-494a-a08b-c07a647733f4  
  1623732669  
  8713087228  
  1623732669  
  </code></pre>
  
  <p>I need to know which <code>cn</code> has double entries  </p>
  
  <p>A <code>cn</code> could have more than one entry.  </p>
  
  <p>So the <code>cn</code> <code>8d7d1204-4d5b-48ab-8f2c-3ab5d7da2d3d</code> has two double entries in the example above:  </p>
  
  <pre><code>3339625694  
  3546846448  
  3339625694  
  3546846448  
  </code></pre>