Emacs 24.4 introduces the command delete-duplicate-lines.

By default M-x delete-duplicate-lines will delete all but one copy of any identical lines in the region. So assuming this is the region:

1
2
3
4
5
6
7
8
9
10

test
dup
dup
one
two
one
three
one
test
five

it will be reduced to this:

1
2
3
4
5
6

test
dup
one
two
three
five

When the command is invoked with a C-u prefix, it searches backwards and keeps the last instance of each repeated line. So this:

1
2
3
4
5
6
7
8
9
10

test
dup
dup
one
two
one
three
one
test
five

will become:

1
2
3
4
5
6

dup
two
three
one
test
five

Identical lines need not be adjacent, unless you’ve invoked the command with a C-u C-u prefix. When the command is invoked with a C-u C-u C-u prefix, it retains repeated blank lines.