I may be the only person on the planet with WinPhone, Groups but this after scheduling a series of meetings in OWA Calendar for some of my Groups, my phone went beserk as it had 20+ copies of the same calendar entry (I suspect one for each member of the Group) on my phone - so that was 20 alarms and reminders! (@Dona Sarkar?)
Windows 10 Mobile 15207
Calendar app 17.8117,10002.0