In an Excel workbook, I've got data in one column on two worksheets. The second worksheet contains old data. New data is entered in the first worksheet. I need to avoid duplicate entries, i.e. if I try to enter a phone number that is already on either sheet, it will not allow the phone number to be added.

I have no problem accomplishing this with a COUNTIF formula in data validation, but I can't seem to figure out how to check to see if it's on the second worksheet.

New numbers are constantly being added and it can't be something where you have to go back and run a search afterwards, it needs to notify the user that the number exists right when it is entered.

Any help is greatly appreciated!

-Chris