Twins are not too uncommon, even without fertility drugs. There are two ways you can naturally end up with twins.

One way is that the woman's body releases two eggs instead of one. That means during sex, two sperm can come in, one for each egg. That makes two babies called fraternal twins. They may be female/female, male/male, or female/male. They will look about as similar as any two regular siblings.

The other way is that one egg meets one sperm, but early on in development, things just get split in half, developing into two babies with the same DNA. These are called identical twins. They will be male/male or female/female. Those are the kinds of twins that you can't always tell apart easily.