The "Open With..." option in the Finder's contextual menu (or in the "info" window for documents) may sometimes list incorrect numbers of relevant applications. When this happens, the menu may show duplicates of all entries, or may only show doubles of certain applications. Additionally, applications that could open a specific document may be missing from the menu.
If you have problems with this menu, first check to be sure you do not have duplicate applications installed on your system. Sometimes updates or reinstalled applications may not replace old files but instead leave both the new and old copies on the system. If you do have this, then both may show up in the menu.
The next step after ensuring you only have one copy of an application installed, is to reset the launch services, which is the service that pairs up file types and URLs with handler applications. This can be done two ways: manually or with a third-party utility.
To reset the services manually, open the Terminal and run the following command (the utility to do this is buried deep in the LaunchServices framework, and therefore the commands below are long):
For Leopard and Snow Leopard:
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -seed -rfv -all user, system, local
For Tiger and previous:
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user
You might also try removing the launch services preference files, which are the "com.apple.LaunchServices.plist" and "com.apple.LaunchServices.QuarantineEvents" in your /username/Library/Preferences/ folder. Do this and then run the commands to clear and rebuild the launch services.
If you are unsure about using the Terminal to run these commands, you can try a third-party utility such as OnyX (Available as a public beta for Snow Leopard) or another cleaning and maintenance utility that will clear and rebuild the Launch Services.