title: Pandas Dataframe: how to add column with number of occurrences in other column
body: <p>I have to following df:</p>
  
  <pre><code>Col1    Col2
  test    Something
  test2   Something
  test3   Something
  test    Something
  test2   Something
  test5   Something
  </code></pre>
  
  <p>I want to get</p>
  
  <pre><code>Col1    Col2          Occur
  test    Something     2
  test2   Something     2
  test3   Something     1
  test    Something     2
  test2   Something     2
  test5   Something     1
  </code></pre>
  
  <p>I've tried to use:</p>
  
  <pre><code>df["Occur"] = df["Col1"].value_counts()
  </code></pre>
  
  <p>But it didn't help. I've got Occur column full of 'NaN'</p>