title: Keep only groups of data with multiple observations
body: <p>I am attempting to keep only deids with multiple observations. </p>
  
  <p>I have the below code</p>
  
  <pre><code>help &lt;- data.frame(deid = c(1, 5, 5, 5, 5, 5, 5, 12, 12, 12, 12),
                     session.number = c(1, 1, 2, 3, 4, 5, 6, 1, 2, 3, 4),
                     days.since.last = c(0, 0, 7, 14, 93, 5, 102, 0, 21, 104, 4))
  
     deid session.number days.since.last
  1     1              1               0
  2     5              1               0
  3     5              2               7
  4     5              3              14
  5     5              4              93
  6     5              5               5
  7     5              6             102
  8    12              1               0
  9    12              2              21
  10   12              3             104
  11   12              4               4
  </code></pre>
  
  <p>My feeble attempt was to use the group_by and then the filter( ) command</p>
  
  <pre><code>help %&gt;% group_by(deid) %&gt;% filter(session.number &gt;=2)
  </code></pre>
  
  <p>However, it only keeps session.number's at 2 or greater. So I get rid of the deid = 1, but all the remaining deid data starts at session.number 2, and not session.number 1. </p>
  
  <p>What I am trying to tell R is to keep the groups (deid) with greater than 1 observation (session.number)</p>
  
  <p>Any assistance is greatly appreciated. </p>