About Duplicate Fieldnames
If you define an SQL statement to create a data source by joining tables, be sure to account for fields with the same name in the source tables. Fields with the same name will cause problems if you try to copy the new data source.
For example, this SQL statement creates two LastName fields in the new data source:
select $A$.EMail_, $A$.LastName, $B$.LastName from $A$ inner join $B$ on ($A$.EMail_ = $B$.EMail_)
Instead, use this SQL statement, which solves the problem by specifying aliases that make the field names unique:
select $A$.EMail_, $A$.LastName AS LastNameA, $B$.LastName AS LastNameB from $A$ inner join $B$ on ($A$.EMail_ = $B$.EMail_)