title: Find indexes of duplicate strings in C# List
body: <p>I have a List of strings:</p>
  
  <pre><code>["String1"]
  ["String2"]
  ["String1"]
  ["String3"]
  ["String2"]
  ["String1"]
  </code></pre>
  
  <p>That I need to search and find the indexes of "String1" in the List and also count how many times "String1" occurred.  I've reviewed <a href="https://stackoverflow.com/questions/2718822/what-is-the-most-elegant-way-to-find-index-of-duplicate-items-in-c-sharp-list">this answer</a> but I'm new to this type of coding in C# and I'm unclear how to extract the index values, so if you could explain how to use the solution, that'd be great!</p>