Duplicate 365 accounts with same email address

Hello, we are a medium sized MSP and have started to roll out 365 (primarily for Teams) for our customers. Some of these have had personal 365 accounts for testing or other reasons before we sync their accounts via Azure AD Connect from the customer OU to a new 365tenant. If these older personal accounts were created with the same email address that now gets synced to Azure AD, Teams will not logout the old account and creating meetings will fail. The only solution we found so far is to manually change the primary email address in the old personal account, remove the business email address and wait a day or so until everything works as it should. Is there a way to check if accounts already exist with an email address in our exchange environment/local AD? Has anyone found a better way to separate these accounts?

Thanks, kind regards

Martin