You might expect the following:
   new RedirectResolution("/foo.jsp").addParameter("bar", "one").addParameter("bar", "two");

to produce a redirect to "/foo.jsp?bar=one&bar=two", but it's actually just overriding it to "/foo.jsp?bar=two".

It's not as horrible as it might sound, because the workaround is just to use the var-arg method call: addParameter("bar", "one", "two"), but it would be better and convenient if ti worked for multiple calls.