I haven't proven this correct yet, but I am thinking redundant data in two tables makes sense if you need that data to join the tables properly and avoid table/index scans in the execution plan. You could create indexes on the redundant columns to force index seeks on both tables.
My initial reaction to this statement was going to be "well that explains the percentage of incorrect answers".
But to answer more seriously, I'd say it sounds like another edge case scenario. In general, I would try to use a primary key foreign key relationship to join the tables, or a cross-reference table that uses a key from each table make the connection, or any other method that doesn't involve storing, indexing and joining on redundant data with no constraints that ensure data integrity.