It happens on a new playthough.
The problem is "$ my_events.add("mother_event_1")" is only entered once outside of the event and the event can play twice.
To replicate it:
1 - Don't do anything with the nurse. No blowjobs for you :P
2 - Follow main story line so "principal_event_3" gets added to my_event.
3 - In "label principal_event_3.part_3" without a blowjob from the nurse it will trigger 'mother_event_1'
4 - Wait until Night at MC House Entrance. This checks if "principal_event_3" was entered into my_event and after checks if "mother_event_1" was entered. Going with the no nurse love for you route it won't be in the list.
5 - This triggers 'mother_event_1' to play again. Other than scoring more love/hate with Akari it's not game breaking just odd that it plays twice.