Did some testing with Office 365, sending invites from Google Calendar @gmail account (web):
Test 1:
- Sent invite, received in Office 365 / web, "accepted" from message (not calendar event)
--> Invite moved to Deleted, Office 365 sent "invite accepted", meeting marked with "you accepted on ..." in Office 365 Calendar
- Updated in Google Calendar (organizer), Office 365 received "updated: ..." email.
--> At this point, the meeting is no longer marked as "you accepted on ..." in Office 365 Calendar; In the new email (about the update), all buttons including Accept are active in both Office 365 mail and AquaMail
Test 2:
Same as Test 1, but I accepted the original invite in AquaMail.
This time I also "accepted" on the "updated: ...." email in AquaMail.
This caused this message to be moved to Deleted, a response to be sent, and also the calendar item in Office 365 / web calendar was again marked with "accepted on...".
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In other words, can't reproduce.
The way this works is Exchange *server* should automatically clear the "already accepted" marker on the calendar item when it receives the "meeting updated" notification. It does for me, in Office 365.
Maybe that's the difference here? A debug log (of a full cycle from original to updated) might help.