asp.net mvc - ReturnUrl Duplicating Parameters - MVC 4 -

i'm having problem retunurl duplicating querystring parameters.

my url like:

"www.mysite.com/order/?id=1&item=123"

then, redirect me login page , url like:

"www.mysite.com/login/redirecturl=/order?id=1&item=123&id=1&item=123"

after user login, action redirect to:

"www.mysite.com/order/?id=1&item=123&id=1&item=123"

in page when use request.querystring["id"] got error, because querystring "id" duplicated.

my login action code this:

[httppost] [allowanonymous] public actionresult index(loginmodel model, string returnurl) {     if(verifylogin(model))     {        if(returnurl != null)           return redirect(returnurl);//redirect url duplicated parameters        else           return redirect("/home");     }     else     {        modelstate.addmodelerror("", "invalid username or password");     }     return view(); } 

how can solve problem?

i think problem in order controller or returnurl assignment logic. might adding url+querystring. if yes can try request.url.getleftpart(uripartial.path) + querystring