title: dask: groupby on bag shows unexpected behavior
body: <p>I'm going through the dask tutorial at <a href="https://tutorial.dask.org/02_bag.html" rel="nofollow noreferrer">https://tutorial.dask.org/02_bag.html</a> and am surprised by the behavior of groupby + starmap on a bag:</p>
  <blockquote>
  <p><a href="https://i.stack.imgur.com/EK7mC.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/EK7mC.png" alt="Screenshot Dask Tutorial" /></a></p>
  </blockquote>
  <p>Why are there multiple entries with the same key in the first cell's result?
  The author did not seem to expect that either: They say the 2nd cell produces the same result. It doesn't.</p>
  <p>Did something change in the definition of groupby?
  If this behavior is expected (just not by me), is it documented somewhere?
  I tried and failed to reproduce this with a bag created from a sequence.</p>