I am trying to find a way for Excel to alert me if a series of numbers is
entered twice in the spreadsheet. Currently, I have a spreadsheet that
resembles this:

Column A Column B
Surname, First ID #

The problem is that I don't want to enter "Smith, John" 123456 in Row 15
and then accidentally enter the same name and ID again in Row 300 for
example. The name will (occasionally) give me the black
highlight/autofill-in if the program recognizes the name, however, it isn't
consistent in doing this. Plus, I may be entering the same name with a
different ID (ex: Smith, John 654321). Is there some way for Excel to let
me know if I've entered that ID in a prior row, or do I have to tediously
search each ID before entering it?

I'm not very Excel savvy, so any info anyone provides would be greatly
appreciated, however, please put it in simple terms! Thanks!!