There are some scripts out there for how to add custom code to your report, you can also handle this through doing some SSRS grouping but each of these options will be painfully more difficult than it should be to get a unique set of values.

As has been said, the only good way to do this is by changing the underlying SQL. There's no reason that should not be an option. As ivan said, you could just use the existing query as a subquery and run:

select distinct a,b,c,d from (subquery) A