title: pandas: difference between two series return None
body: <p>Hi I'm trying to find the difference between two series but it return an empty list. </p>
  
  <pre><code>new_main_file = pd.read_excel('result_concat.xlsx', encoding='utf-8')
  new_main_file.Title.count()#=&gt; 11 620
  len(new_main_file.Title.unique())#=&gt; 10 436
  
  #Difference
  pd.Series(list(set(new_main_file.Title) - set(new_main_file.Title.unique())))
  #Series([], dtype: float64)
  </code></pre>
  
  <p>I am trying to find which title are in duplicate</p>