When adding a new shared mailbox to Outlook for Mac after you have been given access to it, you may be seeing "Account already exists" error message.
This error occurs because you’ve tried to add a shared mailbox through the main “Add email account” window. While Microsoft is working on fixing this issue, the following workaround is available. Close the error window and return to the main Accounts window. If you haven’t added your main email account, you’ll need to before you can add the shared mailbox. Once your main email account has been added:
- Select Delegation and Sharing.
- Select Shared with me.
- Select the plus (+) in the lower-left corner of the window.
- Add the shared mailbox.
Source: Outlook for Mac “Account already exists” error - by Microsoft Support