Your problem is discussed here: Removing Duplicated Data after Joining Tables | Tableau Software
I prefer data blending in these scenarios. If you can post another version of the workbook with the data sources connected separately, I can try to show you how to set that up.
This is useful. Actually I was trying to resolve this without creating another connection but the examples in the link you've provided quite helpful and yes I can manage to do this with a separate connection.
You may be able to do everything you need in one data source, but if data blending resolves the problem and makes sense to the analysis, it is an excellent feature of Tableau.
Cheers! If you need more help, just post another question.