Find a duplicate invoice number within two tables with a message box?

[NewToAccess1016]

I have two tables "Invoice Log 2012" & "Invoice Log 2013"...I need to know how to create a message box in the "Invoice Log 2013" when I am entering a new record to tell me if the invoice number I am entering is a duplicate entry from either of the two logs. I need the option to either accept the entry and continue or reject the entry. Again, I am new to this. If anyone can help, I would appreciate it immensely. I need a step by step instruction for this.

[RuralGuy]

You could use code in the BeforeUpdate event of the InvoiceNumber control to do DLookup's in the two tables.