Is it normal having one boob bigger than the other one?

Most grown women have one breast that is slightly larger than the other. It's perfectly normal. You're only 12 years old -- give 'em time to grow! In the meantime, you can wear a lightly padded bra under your fitted t-shirts, and then nobody will be able to tell the difference.

Totally normal. It's rare for them to be identical.

yes

It must be because this question gets asked every day, multiple times a day.

Yes, it's completely normal to have breasts that are different sizes.

This is true for all women, we're simply not meant to be symmetrical and our breasts develop independently - throughout puberty you may notice a more significant difference because often your breasts will grow differently and at different speeds. It is a little annoying but it's totally normal and something we all experience in our lives.

Abreast: The Basics of Boobies - http://www.scarleteen.com/article/advice/abreast_the_basics_of_boobies
Frankenboobs - http://www.scarleteen.com/article/bodies/frankenboobs
Normal Breasts Gallery (NSFW) - http://www.007b.com/breast_gallery.php

It is normal to have a slight difference in size.
A huge difference is a medical condition, and it can be treated by a plastic surgeon when you reach adulthood.

Yes. Sometimes the difference is barely noticeable but for example in my case it's very clear. My left breast is an A cup and my right one is a B cup.

You're still in your teens so your breasts have a lot of time to develop still.