Duplicated and appended URL

The %20 character is the html replacement for a space.
Check your code if your base url or the url you are trying to redirect to, start or end with a space character (by mistake).
I use the redirect statement in a lot of situations. I've never encountered any problems when redirecting to a url that refers to another controller. It could go wrong when your controllers are in subfolders. But I don't see that in your description.