IE. input.csv

employid, start, stop, total
1234, 01/04/2013, 05/04/2013, 5
1234, 08/04/2013, 12/04/2013, 5
1234, 18/04/2013, 18/04/2013, 1
5678, 01/01/2013, 01/01/2013, 1

  output.csv

employid, start, stop, total
1234, 01/04/2013, 12/04/2013, 10
1234, 18/04/2013, 18/04/2013, 1