I had a failed attempt at importing my data into Connections, so I deleted all the contacts and started again. I now have a (more or less) clean list assigned to various mailing lists but this has brought about its own issues.
My suggestions are:
1) I have a complete set of duplicates (and more - 832 people in Contacts but 1,705 people on the 'Everyone' list) with no reassurance that, if I use this list, it won't send each mail twice!
2) Provide an easier way of searching for duplicates in the contacts list. At present all you seem able to do is browse down a long list and try to spot them yourself (I should send you my optician bill!!!)
3) For GDPR purposes I want to remove any phone numbers I added in ignorance - I don't have a need to keep these within MS Connections. But again, I have no way other than to browse down each list, edit each contact and then repeat, as saving a contact makes the list scroll back to the top each time.
As a related issue, if you have multiple contacts with the same email address (eg husband and wife) Connections complains when you try to add both to a mailing list - whereas actually you may need to send out separate opt-in emails to each person individually ...