Receiving this error usually indicates that the same work email address has been added for more than one employee.
To check if this is the case, run a Full Employee Data Extract report:
Go to Reports and click Standard Reports.
Select Data Extract and then Full Employee Data Extract Report.
Select the entire organisational structure and click Generate Extract.
Open the report that has been downloaded.
Press CTRL + F on your keyboard or click Find.
Search for the employee's work email address.
This will find all instances of the work email address on your system. Check if this has been added to any incorrect employees.
Once you find which employees have an incorrect email address, change it by following the steps in our article on amending work email addresses.