title: Count rows with same dates
body: <p>I have the following variable:</p>
  
  <pre><code>     datevote 
  1   1981-6-21
  2   1981-6-21
  3   1981-9-23
  4   1982-2-14
  5   1982-2-14
  </code></pre>
  
  <p>and so on...</p>
  
  <p>What I'd like to have in the end is a new column in which the number of the same dates of column "datevote" ist displayed:</p>
  
  <pre><code>  datevote        numbervotes
  1   1981-6-21     2
  2   1981-6-21     2
  3   1981-9-23     1
  4   1982-2-14     2
  5   1982-2-14     2
  </code></pre>
  
  <p>I have tried several things, but it seems more work than expected. Can anyone help me?
  All the help is appreciated, thanks.</p>