title: Find number of occurrences in different variables
body: <p>I have a data frame with some data about different people. They look like this:</p>
  
  <pre><code>Year Item ID
  2005   a   1234
  2005   b   1234
  2005   a   4567
  2005   b   4567
  2006   a   4567
  2006   a   7894
  </code></pre>
  
  <p>My data has 45000 observations and about 1000 different ID's and 10 different years. I want to find the ID's of people that appear in more than 1 year, how do i do this? I thought of separating the data by ID and seeing if the resulting data has different years, but that doesn´t seem like the smartest way to do that</p>