I was just testing an import today and noticed the duplication import issue. This is on an IMAP account that is no longer active.
In M2, I copied one email to another IMAP account as a backup and imported that account first. So the copied email exists in M3 on import.
However, when I import the original account, then all the email from the original account is successfully imported, but the email that I copied to the other account does not get imported in with the original account.
On test install, if I only import the original account, then the email is imported as expected.