Build a table from a list by comparing existing table entries

I am new to this shell scripting....

I have a file which contains list of users. This files get updated when new user comes into the system.

I want to create script which will give a table containing unique list of users. When I say unique, it means script should match table while parsing user file list.

E.g.

List-----
User1
User2
User1
User3
User4
User1

The script output should be a list with

User1
User2
User3
User4

Thanks

sort list.txt | uniq

Or:

awk '!a[$0]++' file

Thanks mjf.

Your suggestion solved my purpose.
Thanks for prompt answer.