title: Convert List of tuple to map (and deal with duplicate key ?)
body: <p>I was thinking about a nice way to convert a List of tuple with duplicate key <code>[("a","b"),("c","d"),("a","f")]</code> into map <code>("a" -&gt; ["b", "f"], "c" -&gt; ["d"])</code>. Normally (in python), I'd create an empty map and for-loop over the list and check for duplicate key. But I am looking for something more scala-ish and clever solution here.</p>
  
  <p>btw, actual type of key-value I use here is <code>(Int, Node)</code> and I want to turn into a map of <code>(Int -&gt; NodeSeq)</code></p>