Number of rows changes even after `pandas.merge` with `left` option : <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>
