Forms :: Only One Record Can Be Current

I have a table with fields for the year, dues, and a Boolean field that indicates whether or not its record is the current year record.

I have a form based on the table that is used to update the table.

My business rule is that only one record can have the "current year" field checked.

I am working with the Form_Before_Update event. What I would like to happen is if the person selects the "current year" check box and if there is already another record with its check box selected, my code will realize that the user has selected a second record and undo the check box and cancel the event.

Question: Looking for code that would check for the second record being checked and back out the user's check for that record?

Note: I have to account for not only the user clicking the record selector to try and save the record, but also the case where they check the box, then click on the "close form" button that I created.