Hi @Bob_Adams,
Really appreciate you raising this request, with detailed examples and analysis! 
Indeed as you shared, right now the Sum Rows in Data Table is only calculating on the visualization result. Therefore, it would count in any duplicated rows.
Ensuring the Sum Row’s aggregation to consider unique records would indeed provide a clearer and more reliable view of the data, especially in your example.
Let me check with our team about this feature. Will keep you posted on any updates.
Cheers!