Problem: Employee is getting multiple invites /emails for users. This is due the employee(whose sending the invites /reply to calendar) having an exchange account on iphone being existent before our migration (about a year ago).
This is most commonly seen with Iphone users that have their email setup on their Iphone.
Solution: If the employee removes the exchange account from their phone and then adds it back. It should then work normally and calendar invites show regularly.