For example, if there are 20 links to dir/index.htm and only two to /dir/, Google will most likely list dir/index.htm, and in my mind, that would be the version you would want to keep for minimum disruption to your rankings. If it's 20 for /dir/ and two for dir/index.htm, then keep /dir/ and bag the other.

The other consideration would be if there are external links to the page using one form or the other -- external links are harder to change, and are potentially more valuable.

Whichever version you decide to keep, do the 301 from the version you don't keep to the version you do, and change all the 'wrong' form links on your site the the 'right form'.

HTH

I've made 301 redirection for some pages, but I couldn't visit those pages using IE, and the firefox showed "Firefox has detected that the server is redirecting the request for this address in a way that will never complete."
What's wrong? Any response will be appreciated.

Somewhere in the deep recesses of WebmasterWorld there is a thread or two on this matter. A google site search might find it for you.

I don't know of a native solution in IIS - I've heard you can do it with ISAPI Rewrite, but that's a third party helper application.

RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /([^/]*/)*index\.(html?Šphp)\ HTTP/
RewriteRule ^(([^/]*/)*)index\.(html?Šphp)$ http://www.example.com/$1 [R=301,L]

RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /([^/]*/)*default\.asp\ HTTP/
RewriteRule ^(([^/]*/)*)default\.asp$ http://www.example.com/$1 [R=301,L]

You must be more carefull, that has been repeated so many times that WebmasterWorld will get dup dinged ;-).