[R] How to remove rows representing concurrent sessions from data.frame?

[johannes rara]

I have a dataset like this (dput for this below) which represents user
computer sessions:

   username          machine               start                 end
1     user1 D5599.domain.com 2011-01-03 09:44:18 2011-01-03 09:47:27
2     user1 D5599.domain.com 2011-01-03 09:46:29 2011-01-03 10:09:16
3     user1 D5599.domain.com 2011-01-03 14:07:36 2011-01-03 14:56:17
4     user1 D5599.domain.com 2011-01-05 15:03:17 2011-01-05 15:23:15
5     user1 D5599.domain.com 2011-02-14 14:33:39 2011-02-14 14:40:16
6     user1 D5599.domain.com 2011-02-23 13:54:30 2011-02-23 13:58:23
7     user1 D5599.domain.com 2011-03-21 10:10:18 2011-03-21 10:32:22
8     user1 D5645.domain.com 2011-06-09 10:12:41 2011-06-09 10:58:59
9     user1 D5682.domain.com 2011-01-03 12:03:45 2011-01-03 12:29:43
10    USER2 D5682.domain.com 2011-01-12 14:26:05 2011-01-12 14:32:53
11    USER2 D5682.domain.com 2011-01-17 15:06:19 2011-01-17 15:44:22
12    USER2 D5682.domain.com 2011-01-18 15:07:30 2011-01-18 15:42:43
13    USER2 D5682.domain.com 2011-01-25 15:20:55 2011-01-25 15:24:38
14    USER2 D5682.domain.com 2011-02-14 15:03:00 2011-02-14 15:07:43
15    USER2 D5682.domain.com 2011-02-14 14:59:23 2011-02-14 15:14:47
>

There may be serveral concurrent sessions for same username from the
same computer. How can I remove those rows so that only one sessions
is left for this data? I have no idea how to do this, maybe using
difftime?

-J

structure(list(username = c("user1", "user1", "user1",
"user1", "user1", "user1", "user1", "user1",
"user1", "USER2", "USER2", "USER2", "USER2", "USER2", "USER2"
), machine = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 3L,
3L, 3L, 3L, 3L, 3L, 3L), .Label = c("D5599.domain.com", "D5645.domain.com",
"D5682.domain.com", "D5686.domain.com", "D5694.domain.com", "D5696.domain.com",
"D5772.domain.com", "D5772.domain.com", "D5847.domain.com", "D5855.domain.com",
"D5871.domain.com", "D5927.domain.com", "D5927.domain.com", "D5952.domain.com",
"D5993.domain.com", "D6012.domain.com", "D6048.domain.com", "D6077.domain.com",
"D5688.domain.com", "D5815.domain.com", "D6106.domain.com", "D6128.domain.com"
), class = "factor"), start = structure(c(1294040658, 1294040789,
1294056456, 1294232597, 1297686819, 1298462070, 1300695018, 1307603561,
1294049025, 1294835165, 1295269579, 1295356050, 1295961655, 1297688580,
1297688363), class = c("POSIXct", "POSIXt"), tzone = ""), end =
structure(c(1294040847,
1294042156, 1294059377, 1294233795, 1297687216, 1298462303, 1300696342,
1307606339, 1294050583, 1294835573, 1295271862, 1295358163, 1295961878,
1297688863, 1297689287), class = c("POSIXct", "POSIXt"), tzone = "")),
.Names = c("username",
"machine", "start", "end"), row.names = c(NA, 15L), class = "data.frame")