Report Parameter Available Values?

I am in the process of building some custom TFS Reports in Report Builder 3.0. I am building the reports from TFS Analysis Database. I am trying to figure out how to setup a parameter so that available values aren't repeated in a filter drop down. 

Here is what I have so far:

"Iteration" Parameter Properties:

"Iteration" available values Parameter Properties:

I have a filter setup on Iteration Path using expression below:

This works but there is a problem with the same available values being available in the drop down more than once. I am working on a TFS bug report so the report returns all TFS bugs. There obviously are many bugs under the same iteration which is why the same iteration is being repeating as an available option.

How can I prevent duplicates showing under available values?

Thanks,