uniq2 [OPTIONS] [INPUT [OUTPUT]]
OPTIONS
    -a, --adjacent        delete only adjacent duplicated lines.
    -d, --delete-lines    only prints deleted lines.
    -i, --ignore-case     case sensitive.
    -h, --help            print this message.

INPUT                     gives file name of input.  If argument is single dash ('-')
                          or absent, the program read strings from stdin.
OUTPUT                    represents the destination.

$ cat -n testdata/test1.txt
1	a1
2	a1  # <- is the duplicate of the previous line.
3	a2
4	a2  # <- is the duplicate of the previous line.
5	a3
6	a4
7	a1  # <- is the duplicate of the first line.
8	A1
$ uniq2 testdata/test1.txt
a1
a2
a3
a4
A1
$ uniq2 -a testdata/test1.txt  # same result as uniq command.
a1
a2
a3
a4
a1  # <- this line is not deleted.
A1
$ uniq2 -i testdata/test1.txt # ignore case
a1
a2
a3
a4
$ uniq2 -d testdata/test1.txt # print delete lines.
a1
a2
a1

export PATH=$(echo $PATH | tr : '\n' | uniq2 | paste -s -d : -)