Airline websites are designed to automate every interaction. No human interaction. But when they make a mistake, you have to talk with them on the phone. But there's a 35 minute wait. United is going to cancel my reservation if I don't show up on a flight from Las Vegas to San Francisco on Tuesday night. It seems that for situations like this they should be ready to work with customers via email.
Update: I tweeted the problem in 140 chars to the @united account on Twitter. They asked for the flight number and confirmation number via DM, and once they had that they wiped the extra flight from my reservation. It's a secret that you often can get better service by going through Twitter.