For unknown reason, multiple records with the same section number exist within the single course (Moodle 1.9.5):

db=> select id, section, sequence, visible from mdl_course_sections where course = 9630 order by section;
id | section | sequence | visible
-------------------------------------------------------+--------
126077 | 0 | | 1
132598 | 1 | | 1
132579 | 1 | | 1
132564 | 1 | | 1
126078 | 1 | | 1
126088 | 1 | | 1
132599 | 2 | | 1
132580 | 2 | | 1
132565 | 2 | | 1
126079 | 2 | | 1
126089 | 2 | | 1
132600 | 3 | | 1
132581 | 3 | | 1
132566 | 3 | | 1
126090 | 3 | | 1
126080 | 3 | | 1
126081 | 4 | 242363,252739,252740,243288,252754,255210 | 1
132601 | 4 | | 1
132582 | 4 | | 1
126091 | 4 | | 1
132602 | 5 | | 1
132583 | 5 | | 1
126092 | 5 | 242367,242371,252765 | 1
126082 | 5 | | 1
132603 | 6 | | 1
132584 | 6 | | 1
126093 | 6 | 252766,252767,252768 | 1
126083 | 6 | | 1

This creates the situation when we can see the sections (in "topics" mode), but can't see any activities inside. And, if I transfer sequences manually into the section with lowest ID and thus make them visible, it's impossible to move the activites up and down after that - they just go to another hidden record and become invisible again.

Putting aside the cause of the issue (which I haven't found yet), is there any reason why such records may legally exist? Wouldn't it be easier just to put a UNIQUE constraint for course+section?