Ok, so I'm fairly new to Excel and am trying to get this figured out.

Basically, I am manually entering information into the spreadsheet. What I am trying to do is have excel evaluate a zip code column (Col B) and add it to another column without duplication.

Example:

Column B Column F
78727 78727
78734 78734
78621 78621
78727 78634
78634 ...
...

I want to be able to add more data (zip codes) to column B, but automatically update everything except duplicate zip codes into Column F.

What's the best way to do that?