​The Primary Key 'X' is Not Unique

Error Example:

An error occurred while executing connection '<connectionName>': Error reading from entity '<entityName>': The primary key '<columnName>' is not unique. There are two or more rows with value ''<value>''. Please make sure to define a unique primary key for synchronization.

Cause:

This is generally caused by having two or more of the same value (or nulls)​ in the column defined as the primary key.

Resolution/Workaround:

The error mentioned which column has the issue and which value (or non-value) is the problem, so you can use that to check the data source and correct the duplicate values.

This is also a variation on Column 'X' is Constrained to be Unique​ so the article for that error may also have useful information.