How to dedupe/merge contacts from different sources?
Merely partially answering my own question:

First make sure to have Settings &gt; Mail, Contacts, Calendars &gt; CONTACTS &gt; Default Account set to Gmail.

Cards from Office 365 and Google that are of the same person can be edited in iOS Contacts. Edit one contact and, right at the bottom (remove the keyboard) one finds link contacts.... There select the other card. (Press link just once.)

This will put all information of that person in one card in iOS Contacts. However, it doesn't seem to push any information from Office 365 to Google or vice versa. If anyone knows how to do that, please let me know.