How do I remove the duplicate lines in this file?

Hey guys, need some help to fix this script. I am trying to remove all the duplicate lines in this file.

I wrote the following script, but does not work. What is the problem?

The output file should only contain five lines:

Later!

You are close.

Try using

sort -u

It works great! Many thanks!

In awk: remove duplicate, nonconsecutive lines

awk '!($0 in a) {a[$0];print}' inputfile.txt
awk '!a[$0]++' infile
perl -ne '$s{$_}++||print' infile