I know this is extremely late, but I want to point out that the "double birth control" advice really depends on what kind of BC you're using. Most doctors I know still recommend that for people who primarily rely on the pill, condoms or other error-prone methods of birth control. It's too easy to forget a pill and not realize it, or take a medication that interferes with oral BC, or put the condom on wrong, etc.

However, for more foolproof methods of birth control like IUDs and Implanon, there's usually not a recommendation for a "backup" method. The reason is that most medications do not reduce their efficacy like they might with pills (the classic example being antibiotics, which can hurt the Pill's absorption in the digestive system), and there's no way to forget them. With those, a secondary method is usually only recommended as a barrier to protect against STIs. There's still a very small chance of pregnancy with IUDs and Implanon, but it's so vanishingly small that it isn't a realistic concern for most people.

That doesn't change the fact that there's nothing wrong with using two methods for peace of mind no matter what hormonal BC you're on, and also that it is absolutely stupid to be having unprotected sex and apparently just hoping you don't get pregnant. I hope the LW and his girlfriend got a clue before they found themselves dealing with an unwanted pregnancy.