If you have an observer to listen to one event, but it also gets triggered when a child event of the expected event is triggered.

Like in my case class grade_item_updated extends grade_item_created. My observer is looking for grade_item_created, but still catches grade_item_updated.

So 

$observers = [

[

'eventname' => '\core\event\grade_item_created',

'callback' => '\local_submissionrestrict\observer::handle_grade_item_created'

]

];

will also catch grade_item_updated.

Steps to replicate:

1. Install https://github.com/catalyst/moodle-local_submissionrestrict
2. Create a course
3. Create an assignment activity.  
4. Trigger duplication of the activity and observe that handle_grade_item_created method is triggered twice in https://github.com/catalyst/moodle-local_submissionrestrict/blob/MOODLE_39_STABLE/classes/observer.php#L40 but events are different type.