How to prevent duplicate entries due to date.

[adriel19pr]

Hello,

I have a data sheet which in column A has the date and in column B the customer number. The customers are not able to repeat on the same date but can come in once a day. I would like a pop up that says the customer already came in today, and prevent it from duplicating the entry due to the date. Here is an example.

Date Customer
4/1/2014 319
4/1/2014 320
4/1/2014 325
4/1/2014 330
4/1/2014 350
4/2/2014 319
4/2/2014 330
4/2/2014 350
4/2/2014 319
4/2/2014 326
4/2/2014 327

I will greatly appreciate all the help given.
Thanks,
adriel19pr

[GC Excel]

Hi,
Here's a suggestion using DataValidation in column B with a formula (no macro)

[adriel19pr]

Thanks GC Excel, You've been of great help.