Remove duplicate entries in a Bash script

Perl one-liner similar to @kev's awk solution:

perl -ne 'print if ! $a{$_}++' input

This variation removes trailing whitespace before comparing:

perl -lne 's/\s*$//; print if ! $a{$_}++' input

This variation edits the file in-place:

perl -i -ne 'print if ! $a{$_}++' input

This variation edits the file in-place, and makes a backup input.bak

perl -i.bak -ne 'print if ! $a{$_}++' input

You can sort then uniq:

$ sort -u input.txt

Or use awk:

$ awk '!a[$0]++' input.txt

It deletes duplicate, consecutive lines from a file (emulates "uniq").
First line in a set of duplicate lines is kept, rest are deleted.

sed '$!N; /^\(.*\)\n\1$/!P; D'