lookup text in a column and say if it exists

I have a spreadsheet that i am distributing that will be used to correct errors in customer info. I have a 6 characer alpha customer ID that I am giving my accounting office the option of changing BUT i want to put a flag up if that ID already exists in that column. example

old id new id note
------------------------
abcdef dfghij *ID Exists* (finds the id on the 5th line)
bcdefg xvfghj
cedfgh asdfer
edfghi poilj1
dfghij abcdef *ID Exists* (finds the id on the 1st line)

this way they can choose another ID.

i have tried using lookup, and hlookup but im aftraid im just not wrapping my brain around this problem.

any help would be appreciated.

thanks in advance!

thanks! works great. i have yet to really use 'match' so i never think about it.