I am afraid I do not know how exactly to reproduce this - but I know for sure this course was created on 2.8 and without any fancy hacks. An error was experienced by Helen when bulk-deleting courses on learn.moodle.net and then once I had worked out which course was affected - I could reproduce the error by visiting the gradebook of the course in question:

"Found more than one record in fetch() !"

Unfortunately that error message provides absolutely no details... But once I hacked the code I found the problematic record.

  id  | courseid | parent | depth |  path  | fullname | aggregation | keephigh | droplow | aggregateonlygraded | aggregateoutcomes | timecreated | timemodified | hidden 

------+----------+--------+-------+--------+----------+-------------+----------+---------+---------------------+-------------------+-------------+--------------+--------

 5579 |     5991 |        |     1 | /5579/ | ?        |           0 |        0 |       0 |                   1 |                 0 |  1425284437 |   1425284438 |      0

 5580 |     5991 |        |     1 | /5580/ | ?        |           0 |        0 |       0 |                   1 |                 0 |  1425284437 |   1425284438 |      0

(2 rows)

The timestamp suggests double click or something??

Anyway what do:

1. The gradebook code could at least provide some details in DEBUG_DEVELOPER mode?
2. Maybe there needs to be some double click prevention in the category creation code?
3. Maybe the gradebook should be more fault tolerant when deting records?