Я бы использовал для этого awk:
$ cat tst.awk
(!NF) { # blank line
b = ""; f = 1 # empty buffer, start buffering
}
/-->/ { # timestamp
f = 0 # stop buffering
if (p == $0) { # same timestamp
next # discard buffer, start over
}
p = $0 # save timestamp
printf "%s", b # print buffer
}
f { # buffering enabled
b = (b $0 ORS) # buffer line
next # start over
}
1 # print line
выходы:
$ awk -f tst.awk file
231
00:13:35,230 --> 00:13:37,120
- Oh, my sister got me into it.
232
00:13:37,129 --> 00:13:38,269
- Yeah?
Is that her?
234
00:13:40,049 --> 00:13:41,090
- Yeah.