I've had a few questions around checking if a value is unique in a column.
(Thanks @Ian Thornton )
In v2 you could make this into a reusable function
@Ian Thornton @Chris Allan does the lookup cache the results of the first time the count is run, or does it perform the count for each row? Is this the same, more or less efficient than grouping on the surname field, counting the occurrences and joining the results back to the original?