Can I just point out that you shouldn't ever take 2 pills together, if you've missed one and realise within a few hours then take it and then take the next as normal but you should never take the missed one and your next one at the same time or even close to one another. Taking two doesn't make up for the missed one and it is dangerous. Also the advice about getting the morning after pill doesn't apply if you're on the combined or mini pill, you can't just fill your body with that many hormones. If you've missed a pill then use condoms for the correct amount of days (depends on your pill but it's usually between 5 and 7). If you've missed the pill and forgotten condoms then either don't have sex until you can get some or if it's too late then wait the right amount of weeks and buy a pregnancy test or go to your gp. Don't overdose your body with hormones. Taking contraceptive pills is considered an everyday and casual thing but they can be really harmful to you so always use them with caution just like you would any other medication x
Also you can get pregnant using any form of contraception: condoms, the combined pill, the mini pill, the implant, the morning after pill, the coil, the diaphragm, the ring, the injection. Any of it you are not 100% covered even with perfect use. So you have to be prepared that you may get pregnant even with contraception. However, the chance is very small so if you've used your contraception properly then don't worry about it unless you are showing signs of being pregnant.
Also the pill doesn't thicken the lining of the uterus, it thins it, but it does thicken the cervical mucus.