...

zuse:Downloads db692$ cat sys_user_from_test_starts_with_b.csv | awk -F, '{print $1}' | uniq -d | wc
      34      34     271

My concerns about this approach

If you go through the set of colliding netids, they seem to be sequential. This makes me concerned that there are lingering bad accounts in our sys_user table that have not yet collided, but will collide when new netids are created as new people develop relationships with Yale.