Trying this out, it would be nice to provide a contextual message at the point of redirect. eg "This mapping already exists and is a 301 redirect. The canonical version of [URL] is [canonical URL] and it redirects to [redirect]". Probably doesn't need to be done as part of this story/PR. Perhaps we should write a separate story to deal with it?

Part of the problem is that the edit/create mapping page doesn't do a good job of summarising the current state.