Or can FileMaker Pro determine uniqueness from a compound key?
You cannot validate a calc field, but you can create a text field with the appropriate auto-enter option, say,
field1 & field2
If you validate this field as unique, records whose concatenated field contents constitute a duplicate as per this calculation will not be imported.
Another option is to import the data as is, and then use a self-joining relationship with both fields as join predicates. In this scenario, if field X (which can be any field you know to be unique) is not equal to selfJoin::field X, then you have a duplicate. (Or, alternatively, if Count ( selfJoin::field X ) > 1.)