accidently entering the same value into combo box

[Hackcess2007]

Hi all,
I split and implemented my equipment check in check out database probibly before i did enough testing. The problem is I am finding problems and the reasons for them. I made a query to track the movement in and out of the warehouse of our equipment BUT, the end user is able to scan (accidentally) the same barcode number in the combo box during the same transaction. It would be great to use a marco or code to have the second instance of the barcode number open a message box and not allow the second entry saying " This was already scanned in" I am not strong with vba but I see the need to learn. Any help is always very much appreciated by you guys gals.
Thank you and have a great day!

 

[Hackcess2007]

Thanks Uncle Gizmo. I have used the "not in list event" before to clear a value from a combo box after a incorrect value was entered. That way when they close the message box it is all set again to received the next scanned value. Defiantly a good idea!