Re: Duplicate Content vs. Redirects

If you put a page up with those (obsolete) URLs, then you must come to
terms with Google or other search engines finding those pages (eg
through links on other sites...).

I would just have those URLs 301-redirect to the new URLs - the
301-redirect tells the client (search engine) that this url is no
longer valid and that it should use the new one instead. A normal
visitor will not notice (other than being shown the content directly).

Using a javascript redirect (as you mention, after 5 seconds) is not a
good idea: Google sees this as a doorway page and might not like it,
I'm not sure about the other sites. But in anycase, a server-based
301-redirect is the perfect way to cover this situation :-)

Cheers
John