info sed

4.14 Make Duplicate Lines Unique

This is an example of the art of using the N, P and D commands, probably the most difficult to master.

#!/usr/bin/sed -f
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 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

As you can see, we mantain a 2-line window using P and D. This technique is often used in advanced sed scripts.