title: How to count number of duplicate pairs in multimap
body: <p>how can I count the number of duplicate pairs (with same key and value) in a multimap of pairs of integers? </p>
  
  <p>For example, my multimap contains the pairs {(6,2), (6,2), (6,3) and (6,4)} so the duplicate count is 1 as I have 1 duplicate pair in my multimap. I have tried using methods such as find() and count() but to no avail. Any help will be greatly appreciated!</p>