I have run into this on two separate occasions now with two competently different clients/environments.
I have a single user who's contacts have appear to have merged after migrating their mailbox to a new server. Note, there was no merge attempted or wanted. The data that's been merged is random and automatic.
I have attached an image of the issue.
The image in the top left is an image of the wrong user.
The first email address that is listed is correct.
The second address was added automatically and is incorrect.
This this result is across many contacts with random information.
The main problem is when the user sends to this contact, it send to both addresses listed in the contact. Which results in emails unknowingly sent to whatever random address was added to the contact.
Is there a way to fix this and prevent it from happening again?
This example is from an Exchange 2013 to Exchange 2013 cross forest mailbox move and Outlook 2016. I have also witness this from an Exchange 2010 to O365 mailbox move with Outlook 2016.
Is there anything under Linked Contacts? If yes, unlink them. Outlook is bad at linking contacts - it was one of the things I hated about People view in Outlook 2013.
If you open the actual contact form, are these addresses under Email1, Email2, Email3?
How is the sender addressing the message that it's sent to both addresses? That should only happen if they select a contact with multiple addresses (in the original Contact form) and click the Email button in the ribbon or right click and choose Create > Email. If the user uses a view where they see the contact card, they can click on an address to send only to that address.