Querying 2 field unless duplicated

New at this, so please bear with me:

I am looking to merge 2 fields unless duplicated. The fields are in the same
row in the same table Female Surname & Male Surname and I have no problem
when the Surnames are different, I think.. (seems a tad convoluted). e.g.,

FemaleSurname and MaleSurname as per below

Contact Name: IIf(IsNull([Female Surname]),IIf(IsNull([Male Surname]),[Male
Surname]),IIf(IsNull([Male Surname]),[Female Surname],[Male Surname] & " and
" & [Female Surname]))

but where the surnames are the same I would like to show it just once.

Can you advise please
thanks
DC1