The difference between
www.domain.com and
www.domain.com/ is that if you try to access the first one, you are redirected to the second one, but VIA the
defaultservername first. That is, if your defaultservername is
domain.com but you have chosen to to use
www.domain.com then an access to that URL will first be automatically redirected by the server to
domain.com/ before being redirected to
www.domain.com/. This also applies to any links like
www.domain.com/folder where you are redirected to
domain.com/folder/ and then finally to the
www.domain.com/folder/ that you wanted. Always add the trailing / to domain or folder URLs.
The meta noindex tag is of no use here, because all of the content that appears to be at various domains is all being served from the same physical folder on the server. Adding a noindex tag there adds that tag for that page on every domain and subdomain of the site. There is no way to "noindex" just one version. The redirect is the correct thing to do. The redirect says "don't index this URL - do that one over there instead".