It is super common in fundraising to get lists for import that include a mix of new records and duplicate records.  For each list that we import, we need to have a way to generate the list once the import is completed (to include all of the records, including existing and newly created through import.)  The way we have to handle it now is to one-by-one search for the existing constituents, copy their constituent ID into the exception file, do an Import to Update existing records, and add the attribute. This could so easily be handled if there was an exception query created (we would then be able to do a global update), or if the constituent ID was automatically included in the Exception File.