Apologies as I can't attach the workbook because of confidentiality.
So, here's the setup:
Q. Totals of the measures are turning out to be too large. A closer inspection of the data shows that Tableau is multiplying/duplicating data. Why is this so and how can I resolve it?
It's highly likely to be down to your joins. Tableau won't duplicate data but if your joins aren't right that can cause data duplication.