I don't know the answer to your question, but I'm facing a similar issue.

My app is handling Google Calendar and Outlook Calendar integrations. It is creating events for both calendars for every appointment on the system (it is an appointments manager app).

The expected behavior is that the user receives an invitation whether it is using Google or Outlook.

The problem is that sometimes when creating the event for the Outlook Calendar, the user receives an automatic duplicate on it's Google Calendar. I think that's because some users link their Google email on Microsoft so the events get replicated.

Any idea on how to detect if the user email has associated its email on both Google and Microsoft before creating the events?