Keeping track of unique strings in a column

This is what I need to do:

Say I have a Column A containing these values:

Bob
Jim
Jim
Hank
Bob


I need Column B to be updated with the unique values of Column A, like this:

Bob
Jim
Hank

Note that Column B needs to be updated automatically, so that if I add "Fred" to Column A, Column B becomes:

Bob
Jim
Hank
Fred


I hope you can help, this has been eating at me for ages now.
Thanks in advance!