Prevention of Duplicate Field on a Form

[bill crumpton]

I have "Form A" which contains two fields, one of which is a hidden key field and the other is a combobox with 10 predefined choices for the user to choose from. On "Form A" I also have "Subform A" which contains text fields for the user to populate. I need the combobox data in "Form A" not to be able to be duplicated during the same day, however on the next day the same data may be used. For example, on 01/01/00 "data 1" was selected from the combo box on "Form A". On 01/02/00 "data 1" can also be selected from the combo box on "form A", but if a user tries to select "data 1" on 01/01/00 twice I need a message box to advise the user of the error. I would like the information entered into "subform a" to remain on the screen however I do not want the user to be able to exit "form a" without resolving the conflict. Any help much appreciated.