"Hey, what if the condom breaks, i should double layer."
Whywhywhyyyyy does everyone think this a good idea? I've known since my first health class, when I was 11 years old, not to do this.
And there's still a chance of pregnancy even if a couple is on the pill and wearing a condom. If the pill isn't taken exactly on time, or you're on antibiotics, it's pretty much useless. Condoms are effective if they're put on correctly and don't tear; semen can't just escape from the wrapping (in reference to CtR's comments).
It's also good to understand your/your partner's menstrual cycle. You're only fertile roughly in the middle, during ovulation (which occurs about two weeks after a girl's period).