How to specify more then one value for one key in RouteValueDictionary

Question

I need to redirect to a page with multiple values for the same querystring key. like:

?x=abc&x=yz

This is a constraint, I can not change how this service accepts his parameters.

How can this be done using the RouteValueDictionary?

It is not possible to add the same key twice

myRouteValueDictonary.Add('x','abc');
myRouteValueDictonary.Add('x','yz');

Results in an error (of course)

Adding a array of strings did not do it either.

 myRouteValueDictonary.Add('x',new string[] {'abc','xy'});

Resulted in ?x=System.String%5B%5D :-)

I am out of ideas.