If your database supports the syntax, you can simply use SUM(DISTINCT table.measure_column) and it should apply the distinct function. Note, however, that this can result in under-reporting certain values. For example, assume you have two sales orders with the same amount: $100. As long as you include the order number (or primary key order_id or otherwise) you will get a total of $200. But if you remove order number, the sum(distinct) will see two equal values (both $100) and your total will only be $100 instead of the correct answer $200.
What is the real problem you are trying to solve?
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