We have had a few instances of the following scenario and have not been able to find an exact matching issue online.
User A sends out a meeting request to people inside the office and outside the office. Specific User B invited to the meeting accepts the meeting and then unknowingly sends the meeting organizer multiple meeting acceptance emails even after the meeting has happened. User B has editor privileges to User A's calendar and removes themselves from the meeting and the acceptance emails stop.
Both Users A & B have email configured on their smartphones. Our company leverages Office 365 for its email server.