title: Basic grep/sed/awk script to find duplicates
body: <p>I'm starting out with regular expressions and grep and I want to find out how to do this. I have this list:</p>
  
  <pre><code>1. 12493 6530
  2. 12475 5462
  3. 12441 5450
  4. 12413 5258
  5. 12478 4454
  6. 12416 3859
  7. 12480 3761
  8. 12390 3746
  9. 12487 3741
  10. 12476 3557
  ...
  </code></pre>
  
  <p>And I want to get the contents of the middle column only (so NF==2 in awk?). The delimiter here is a space.</p>
  
  <p>I then want to find which numbers are there more than once (duplicates). How would I go about doing that? Thank you, I'm a beginner.</p>