Wrapping Distinct around a union

I have created three summary tables that pulls revenue information from three disparate systems for a consolidated report. Two of the tables have the potential for duplicate data due to a data classification problem that is currently being worked on. In the meantime, we need to create a data set for the user to report from.

The union statement works fine but it returns duplicate rows when the two systems have the same data. I have set all data fields for the three summaries to be the same in SSAS. When I wrap a distinct around the union, I get the same results as if the union was not there.

Here is the DAX:

=DISTINCT(UNION('Table A, 'Table B', 'Table C'))

Is there a better way to accomplish this? It seems straight forward but ... nothing ever is.