What you are trying to accomplish here is to have one resource (either a page or an entire site) redirect a visitor to a completely different page or site, and while doing so tell the visitor's browser that the redirect is either permanent (301) or temporary (302).
Once enabled, an alias provides a nice clean URL for a path on a site. However Drupal does not remove the old path (eg node/1234). The problem is that you now have two URLs representing the same content. This is dangerous territory for duplicate pages whi