You should not accept both "www.mydomain.com/products" and "www.mydomain.com/products/" as valid URLs -- Pick one as the "correct" URL, link only to that URL from your own pages, and redirect the other one to that URL.
Similarly, you should not accept requests for both "www.example.com" and "example.com" if your server resolves both of these to your site.
If you don't take these steps, you are allowing (and paying for) three search-ranking "competitors" to your own site -- and that "competitor" is your very own site!
Lastly, do not mix the use of the "Redirect" or "RedirectMatch" and "RewriteRule" directives. On some sites (such as yours), this immediately causes problems. On others, the problem only shows up after a server upgrade or a change to a new host. In either case, using RewriteRule for all redirects and rewrites, and putting all external redirect rules ahead of internal rewrite rules will prevent these problems.
For your specific problem, no robust and "portable" solution exists using the Redirect and/or RedirectMatch directives; a RewriteRule is required because the solution requires a RewriteCond to differentiate direct client requests from internally-rewritten requests to both avoid the loop you encountered and allow your internal rewrite to work.
Fixing all of these shortcomings results in the following code:
Options +FollowSymLinks
#
RewriteEngine on
#
# Externally redirect only direct client requests for script filepath to new extensionless "products" URL
RewriteCond %{THE_REQUEST} ^[A-Z]+\ /new-products\.php([?#][^\ ]*)?\ HTTP/
RewriteRule ^new-products\.php$ http://www.example.com/products [R=301,L]
#
# Externally redirect all requests for non-canonical products-page
# URL (with trailing slash) to canonical URL (no trailing slash)
RewriteRule ^products/$ http://www.example.com/products [R=301,L]
#
# Externally redirect all requests for non-blank non-canonical hostnames to canonical hostname.
RewriteCond %{HTTP_HOST} !^(www\.example\.com)?$
RewriteRule ^(.*)$ http://www.example.com/$1 [R=301,L]
#
# Internally rewrite requests for canonical extensionless "products" URL-path to products script filepath.
RewriteRule ^products$ /new-products.php [L]
Adjust the first, second, and fourth rules as needed, depending on your choice for the canonical "products" page URL.
Adjust the first three rules as needed, depending on your choice (www or non-www) for the canonical hostname.
These rules in this specific order will fix the problems described above using only a single client redirect to fix all the problems with any client request.
Jim
