It is possible. Some women release more than one egg a cycle. If the woman were to release an egg, get pregnant, then release another egg that got fertilized, then it is possible. Fraternal twins are...
I have heard it's possible but very rare. Myself personally, I was on the shot for 2 years and never had a period during the entire time. I took the last one in October and it's July, I've still had...
No. Since you can only get pregnant during ovulation, and since you do not ovulate while you are pregnant, then no, you cannot get pregnant again until after giving birth. Provided you only have one...