Hi Martin,
AX threw duplicate records error when user click 'vendor settlement for 1099s' in AP, I found this problem or bug due to user made change to Tax1099Field, it threw error while populating 'Tax1099Summary' table. If you look at the method, it loops through the dataset, adding amount together if vendaccount, Tax1099Field are same, until find a different ones, then insert the record. The problem we had is that during the year, user made change to Tax1099Field, it starts with, for example, 'Misc-2', then 'Misc-7', then back to 'Misc-2', then, somehow, original sorting is not working as it supposed. It summarize first set of 'Misc-2', then 'Misc-7', then summarize the second set of 'Misc-2' and try to insert it, got duplicate error. That's why I try to add a 'TransDate' to sorting clause, but it doesn't work either.
As far as the unique index fields(VendAccount, Tax1099Field, Tax1099State), it is because the way how the it populates table 'TAX1099SUMMARY' by summarizing amount under same index value record, those 3 fields have to sort together.