title: Inserting a new row to data frame for each group id
body: <p>I've looked extensively on stack overflow for a solution, but have yet to find one that works for me.
  I have a data frame that looks something like this:</p>
  
  <pre><code>id    time    latitude    longitude
  A     11:10   381746.0    6008345
  A     11:11   381726.2    6008294
  B     10:56   381703.0    6008214
  B     10:57   381679.7    6008134
  C     4:30    381654.4    6008083
  C     4:31    381629.2    6008033
  </code></pre>
  
  <p>I would like to insert a new row at the END of each id. In this row, I would like 'id' and 'time' to be the same as the previous observation. I would like latitude and longitude to be '394681.4' and '6017550' (corresponding to the end location of all id's).</p>
  
  <pre><code>id    time    latitude    longitude
  A     11:10   381746.0    6008345
  A     11:11   381726.2    6008294
  A     11:11   394681.4    6017550
  B     10:56   381703.0    6008214
  B     10:57   381679.7    6008134
  B     10:57   394681.4    6017550
  C     4:30    381654.4    6008083
  C     4:31    381629.2    6008033
  C     4:32    394681.4    6017550
  </code></pre>
  
  <p>Can anyone think of a solution? Dplyr or data table solutions preferred.</p>