Is a http header that I can set in IIS that will essentially tell the users browser that url
"/something/img.gif" and url "/SomeThing/IMG.gif"
are in fact the same thing and that the browser should NOT re-download that resource? I'm running into cache issues where some urls are cased differently, and so users browsers are re-downloading the resource.
URL's are by definition case-sensitive. The problem is that by default, Windows filesystems are not case-sensitive. This is why IIS added that hack, but normally you should not use it. There's probably nothing you can do about the caching issues, because browsers follow the standard and assume different case is a different file. My suggestion would be to fix your website such that it always uses the same case to request things.