There are a few problems with calendar events with multiple events being shown when the activity supports overrides (assign, lesson and quiz).

Default.

1. If you specify a ‘Due date’ an event is created on that day.

User override.

1. If you specify an ‘Allow submissions from’ date AND a ‘Due date’ date an event is created that spans that length of time (assuming its length is less than or equal to ASSIGN_MAX_EVENT_LENGTH), if it exceeds this it will create two separate events.
2. If you specify an ‘Available from’ date an event is created on that day.
3. If you specify a ‘Deadline’ date an event is created on that day.

Group override.

1. If you specify an ‘Allow submissions from’ date AND a ‘Due date’ date an event is created that spans that length of time (assuming its length is less than or equal to ASSIGN_MAX_EVENT_LENGTH), if it exceeds this it will create two separate events.
2. If you specify an ‘Available from’ date an event is created on that day.
3. If you specify a ‘Deadline’ date an event is created on that day.

If you do all the above three a user can see three different events.