gistfile1.txt

csv_data = 
'1235550695,2016-08-30 15:52:20
2345555220,2016-10-20 09:04:59
2345555220,2016-10-20 11:32:58
3455550667,2016-10-15 18:02:03
3455550667,2016-10-15 18:06:45
3455550667,2016-10-21 23:08:32
3455550667,2016-10-21 23:11:24
'

call_array = CSV.parse(csv_data)

call_array.each do |call|
  call.push(DateTime.parse(call[1]).strftime('%s').to_i)
end

call_array.sort_by!{ |p| p[2] }.reverse.uniq { |p| p[0] }