title: Fastest way to check if duplicates exist in a python list / numpy ndarray
body: <p>I want to determine whether or not my list (actually a <code>numpy.ndarray</code>) contains duplicates in the fastest possible execution time. Note that I don't care about removing the duplicates, I simply want to know if there are any.</p>
  
  <p>Note: I'd be extremely surprised if this is not a duplicate, but I've tried my best and can't find one. Closest are <a href="https://stackoverflow.com/questions/89178/in-python-what-is-the-fastest-algorithm-for-removing-duplicates-from-a-list-so">this question</a> and <a href="https://stackoverflow.com/questions/480214/how-do-you-remove-duplicates-from-a-list-whilst-preserving-order">this question</a>, both of which are requesting that the unique list be returned.</p>