title: Keep only duplicates from a DataFrame regarding some field
body: <p>I have this spark DataFrame:</p>
  
  <pre class="lang-python prettyprint-override"><code>+---+-----+------+----+------------+------------+
  | ID|  ID2|Number|Name|Opening_Hour|Closing_Hour|
  +---+-----+------+----+------------+------------+
  |ALT|  QWA|     6|null|    08:59:00|    23:30:00|
  |ALT|AUTRE|     2|null|    08:58:00|    23:29:00|
  |TDR|  QWA|     3|null|    08:57:00|    23:28:00|
  |ALT| TEST|     4|null|    08:56:00|    23:27:00|
  |ALT|  QWA|     6|null|    08:55:00|    23:26:00|
  |ALT|  QWA|     2|null|    08:54:00|    23:25:00|
  |ALT|  QWA|     2|null|    08:53:00|    23:24:00|
  +---+-----+------+----+------------+------------+
  </code></pre>
  
  <p>I want to get a new dataframe with only the lines that are not unique regarding the 3 fields <code>"ID"</code>, <code>"ID2"</code> and <code>"Number"</code>.</p>
  
  <p>It means that I want this DataFrame:</p>
  
  <pre class="lang-python prettyprint-override"><code>+---+-----+------+----+------------+------------+
  | ID|  ID2|Number|Name|Opening_Hour|Closing_Hour|
  +---+-----+------+----+------------+------------+
  |ALT|  QWA|     6|null|    08:59:00|    23:30:00|
  |ALT|  QWA|     2|null|    08:53:00|    23:24:00|
  +---+-----+------+----+------------+------------+
  </code></pre>
  
  <p>Or maybe a dataframe with all the duplicates:</p>
  
  <pre class="lang-python prettyprint-override"><code>+---+-----+------+----+------------+------------+
  | ID|  ID2|Number|Name|Opening_Hour|Closing_Hour|
  +---+-----+------+----+------------+------------+
  |ALT|  QWA|     6|null|    08:59:00|    23:30:00|
  |ALT|  QWA|     6|null|    08:55:00|    23:26:00|
  |ALT|  QWA|     2|null|    08:54:00|    23:25:00|
  |ALT|  QWA|     2|null|    08:53:00|    23:24:00|
  +---+-----+------+----+------------+------------+
  </code></pre>