Analytics Software

Gurus,
I have a multi select box and in choices, it displays all the values (obviously) including redundant values. What I mean by redundant values is it shows the same value a lot many times and my user doesnt want that. He want it to display only distinct values i.e., each different value should be displayed only once.
Possible cause: The joins,etc..
Steps taken: I used the 'SQL Results' in 'Show' for that column in prompt and added the keyword 'distinct'. It didnt work and later realized that its obvious because thats for results and not for choices.
So, the bottom line is how to display distinct values.

Please help.