ITrack/IDispatch error 31439 - The value or range you are adding has already existed

Cause:

This error occurs because the parameter values were initially saved with the report, so Crystal Reports gets confused when you try to enter new values because it thinks those parameter values have already been filled in.

Solution:

Open up the report, kill any previews that might be open, and make sure "File >> Save Data with Report" is unchecked.