You must link to the URL that you want users to see and use. URLs are defined in links.
After you link to the new URL you need two things.
One is a redirect from old URL to new URL. It is there merely for users asking for the old URL after clicking a link in a stale SERP or in their bookmarks.
You also need a rewrite so that when the new URL is requested the server fetches content from from the old server filepath. Note that it fetches it from the old server filepath, not from the old URL.
URLs are used out on the web and contain a domain name. Server filepaths are used inside the server and do not include a domain name. URLs and server filepaths are two different things. They are merely associated by the action of a server.
You need to reverse the direction of your redirect and replace the current redirect with a rewrite.
The new external redirect:
RewriteCond %{HTTP_HOST} ^[A-Z]{3,9}\ ^/content\.php\?sid=282\ HTTP/
RewriteRule ^content\.php$ http://www.example.com/Cool-Page? [R=301,L]
The new internal rewrite:
RewriteRule ^Cool-Page$ /content.php?sid=282 [L]
As for the category duplication, it can be solved in several ways. The cheapskate way is to add the rel="canonical" tag to both pages, where both reference "cat1". Another is to redirect requests for that page in "cat2" to the real page in "cat1". It would be far better to fix the entire system to not reference category in URL at all.
Change the page URLs to no longer include category data in the URL. Link to the page from whatever categories the page appears in. On that page link back to all the categories the page appears in.
[edited by: g1smd at 8:15 pm (utc) on May 5, 2011]
