ParamDisplayValue(?Prompt?) is repeating.......

[vetteheadracer]

I am displaying a load of parameters on a report which are input on a Parameter Screen. If the user uses the back button to return to the prompt screen and then hits the Finish button again then the ParamDisplayValue(?Prompt?) concatenates itself.

i.e. Run once ParamDisplayValue = All
      run twice ParamDisplayValue = All, All
      third time                                = All, All, All

etc.

Doesn't matter if I use ParamValue instead of ParamDisplayValue it does exactly the same thing.

Any bright ideas how to reset the ParamDisplayValue each time the Paramter entry page appears?