Duplicate reminder emails when a user has two or more roles in a course

[isuru89]

Describe the bug (required) There are duplicate reminder emails being sent when a particular user has been assigned to two or more roles (such as, student, teacher, etc).

To Reproduce (required) Steps to reproduce the behavior:

1. Create a user and a course
2. Enrol user to the above course with both Student and Teacher roles.
3. Create an assignment event with a due date.
4. Run local reminders scheduled task
5. The above user will receive duplicate reminder emails. One for role student and other is for teacher.

Expected behavior (required) Only one reminder email should be sent for a user for a single event.

Cron Logs (required) Not required.

Environment (required):

• Moodle version: >=3.5
• Database: any
• PHP Version: >= 7.1