> Obviously I could declare the combination of FieldA, FieldB unique in
> the first instance,

If you do like this, you cannot post duplicate values. I was thinking
that the new column unique index will be used just for duplicate
finding, since the system will find a duplicate value upon update (not
insert), the -803 error being smth like EXISTS, only that it will use
the index and you can do your cursor parsing in one go

HTH

Radu