This AppleScript isn't intelligent, either, because it, too, would copy all the movies it already copied.
The script assumes that "Jeff" is your login name, that the Movies backup folder is called "Movies Backup" and that it exists in the pathway created here for the purposes of the script.
It starts with if/then so that if the backup Movies folder doesn't exist or was deleted, the script wouldn't stall if it couldn't delete the first backup to allow the second one with the same name to duplicate. You'd save it as an application.
tell application "Finder"
if folder "Movies" of folder "Movies Backup" of folder "Jeff" of folder "Users" of disk "External" exists then
delete folder "Movies" of folder "Movies Backup" of folder "Jeff" of folder "Users" of disk "External"
end if
duplicate folder "Movies" of folder "Jeff" of folder "Users" of startup disk to folder "Movies Backup" of folder "Jeff" of folder "Users" of disk "External"
end tell