The problem: ha_partition::read_range_first() could return a record that is 
outside of the scanned range. If that record happened to be in the next 
subsequent range, it would satisfy the WHERE and appear in the output twice.
(we would get it the second time when scanning the next subsequent range)

Fix: 
Made ha_partition::read_range_first() check if the returned recod is within
the scanned range, like other read_range_first() implementations do.