United is offering double miles for all flights (on paid, published fares) in 2004 for new Mileage Plus members.
Caveat, the offer says that it is targeted to recipients of the promotion, but since it’s for new members I’m not certain whether that’s true. You have to enter an email address, perhaps they have a set of email addresses on file to match the offer. But otherwise it should work. Regardless, if you’re joining Mileage Plus, it’s worth a try.