I would like to keep adding values to column A, but using conditional formatting, tell me if that value already exists in column A. I can't find a formula that doesn't error for circular reference. Any ideas? Column A could be hundreds of rows long.

just to try and make things clearer so we understand. are you saying something like having most of the states in column A and if you choose to add Wyoming have it check to see it exists first ?

Make the rule dependent upon a formula such as the following:
=COUNTIF(A:A,A1)>1

Better still use the same rule as a custom data validation and avoid entering a duplicate value in the first place.

Hi Bob. Close. It does not need to check first. Simply turn red if it does already exist in column A. Now we will have 2 occurances of Wyoming and there could be more.

dflak, I already tried that same formula but get a circular reference error. Also, I have to have the duplicate values for other reasons. Thanks.

I tried dflaks formula again and it worked spendidly. I must have typed and extra character or something. Thanks much to both of you and have a great week!