h
:b
# On the last line, print and exit
$b
N
/^\(.*\)\n\1$/ {
# The two lines are identical. Undo the effect of
# the n command.
g
bb
}
# If the @code{N} command had added the last line, print and exit
$b
# The lines are different; print the first and go
# back working on the second.
P
D