Duplicate Values Question

I have a table containing, among others, a field titled
numSerialNumber ("SN") and a field titled strBoardNumber ("BN"). The
SN field is the primary key, and the BN field is required.
Additionally, the BN field must contain a unique value, unless the SN
gets voided. In that case, the BN field can contain "VOID".

Is there any way to allow this, and to have this check at the table
level? Users will sometimes access the table directly to quickly find
a record that needs to be corrected, and I don't want them to
accidentally change the BN field to a duplicate value.

Or, do I need to just implement something at the form level, and tell
users to view the form in data*** view if they need to make
corrections?

Jason
.