Error "Cannot Link User — Duplicate Email Address Found" During Lightning Sync or Einstein Activity Capture User Setup

Overview

When configuring Lightning Sync or Einstein Activity Capture (EAC) sync for a Salesforce user, the Check Status tool may display the error:

"Cannot link user because another user has the same email address."

This error indicates that two or more Salesforce user records in the org share the same email address. Lightning Sync requires each Salesforce user to have a unique email address to establish a one-to-one (1:1) mapping between the Salesforce user and their Microsoft Exchange or Google mailbox account. When multiple user records share the same email address, the sync engine cannot determine which user to link.
Real-Life Example: A shared mailbox address like support@company.com is assigned as the primary email to multiple Salesforce user records. When an administrator attempts to enable Lightning Sync for one of those users, the Check Status tool displays this error because the email address is not unique to a single user.

Steps to Reproduce

1. Navigate to Setup.
2. In the Quick Find box, enter Outlook Integration and Sync or Gmail Integration and Sync, then select it from the results.
3. Expand Let users sync contacts, events, or both between Microsoft Exchange and Salesforce.
4. Click Check Status.
5. Enter the user's name.

Result: The sync status for "Salesforce and Exchange email addresses linked" shows: "Cannot link user because another user has the same email address."