Records showing as exceptions have had duplicate attributes of another type entered at some time in the past. Those attributes have since been marked as "allow only one per record".
To resolve this:
1. Refer to the records listed as exceptions in the validation report
2. Search for and open the constituent record referenced in the validation exception
3. Select the Attributes tab on the record
4. Select the entire Attribute row to be deleted by clicking the box to the left
5. Right click and select delete row
6. After all duplicate attributes have been deleted, complete the attribute import.