title: Get indices of top x equal elements
body: <p>I need to get the indices of all the largest elements who are equal to each other in a sorted list of ints.</p>
  
  <p>So given this list</p>
  
  <pre><code>elements:   {1 , 1, 2, 3, 4, 4, 5, 6, 7, 7, 8, 9, 10, 11, 11, 12, 13, 13, 13}
  index:       0   1  2  3  4  5  6  7  8  9  10 11 12  13  14  15  16  17  18
                                                                    ^   ^   ^
  </code></pre>
  
  <p>I will get this output</p>
  
  <pre><code>{16,17,18}
  </code></pre>
  
  <p>So far I've got </p>
  
  <pre><code>list.Select((x, i) =&gt; new {x, i})
  </code></pre>
  
  <p>To get the indices, but I can't use <code>OrderBy()</code> with <code>First()</code> or <code>Single()</code> because I need all the max elements' indices, not just the very top one. </p>
  
  <p>Is there an elegant way to accomplish this (with LINQ or otherwise)?</p>