Error Refreshing Report, dataset keeps failing

I am recieving Outlook emails informing that an error has occured when processing the dataset. ( See Image ) 

I have had a look in the report and I can see the following error messages:

Data source errorColumn 'Email' in Table 'Employees' contains a duplicate value '[email protected]' and this is not allowed for columns on the one side of a many-to-one relationship or for columns that are used as the primary key of a table.
Cluster URIWABI-UK-SOUTH-redirect.analysis.windows.net
Activity IDf53fc501-3b82-4fec-80dd-94cc00fd441e
Request IDadc6dd29-c9e8-4e54-876f-73e6dcbb05a9

Looking at the dataset, I can only see one record for that email, is there any other way this could be duplicating?

If anyone has seen this before, any suggestion will be helpful, 

Kind Regards, 

Gareth