title: Bigram without repeated words
body: <p>I would like to analyze a text by counting bigrams. Unfortunately my text has plenty of repeated words (like: hello hello) that I don't want to be counted as bigrams.</p>
  <p>My code is the following:</p>
  <pre><code>b = nltk.collocations.BigramCollocationFinder.from_words('this this is is a a test test'.split())
  b.ngram_fd.keys()
  </code></pre>
  <p>that returns:</p>
  <pre><code>&gt;&gt; dict_keys([('this', 'this'), ('this', 'is'), ('is', 'is'), ('is', 'a'), ('a', 'a'), ('a', 'test'), ('test', 'test')])
  </code></pre>
  <p>but I would like the output to be:</p>
  <pre><code>&gt;&gt; [('a', 'test'), ('is', 'a'), ('this', 'is')]
  </code></pre>
  <p>Do you have any suggestion, also using a different library?
  Thank you in advance for any help.
  Francesca</p>