IN FOUND SET.
Is probably the cause of the duplication. The matching is only done against the records that are in the target table's found set. So make sure to do a "show all records" on a layout based on the target table occurrence before doing the next import.
I found the error. The headers of the data are on row 2, not one. I saved a copy to my desktop, deleting that 1st row. The original had 3 new rows and about 25 updated rows. I worked. :)