title: Number of rows changes even after `pandas.merge` with `left` option
body: <p>I am merging two data frames using <code>pandas.merge</code>. Even after specifying <code>how = left</code> option, I found the number of rows of merged data frame is larger than the original. Why does this happen?</p>
  
  <pre><code>panel = pd.read_csv(file1, encoding ='cp932')
  before_len = len(panel)
  
  prof_2000 = pd.read_csv(file2, encoding ='cp932').drop_duplicates()
  
  temp_2000 = pd.merge(panel, prof_2000, left_on='Candidate_u', right_on="name2", how="left")
  
  after_len =  len(temp_2000)
  print(before_len, after_len)
  &gt; 12661 13915
  </code></pre>