Many mammals, including pigs, dogs, rabbits and cats have two uteruses. In these animals, multiple foetuses can grow in each uterus. The foetuses share the placenta, but each one has its own umbilical cord. All primates have single uteruses.
One in about every 2,000 women worldwide has a double uterus. About one in 25,000 women with uterus didelphys gets pregnant with twins, one to each uterus. (Hence the likelihood of any given woman growing two babies in two separate wombs is about one in 50 million).
In the embryonic stage of human development, a female has more than one "uterine horn," or tubes that ultimately fuse into one uterus. In people with this condition, somewhere in the developmental process the tubes didn't come together, most likely because there was an error in the signals cells received instructing them to migrate to certain places.
Some people with uterus didelphys also have two cervices (cervix is the narrow outer end of the uterus at its junction with vagina) and two vaginas, but some only have one vagina. (Most women with two vaginas do not get surgery to fuse them, because one side is typically bigger than the other, so they have intercourse using just that one side)
Most women aren't even aware they have the condition until they become pregnant and get an ultrasound exam. If she gets an ultrasound about eight weeks into her pregnancy (as most ob–gyns recommend), chances are the ultrasound technician would spot the extra womb. But if the woman doesn't get an ultrasound until 20 weeks or more into her pregnancy, the uterus housing the foetus might have grown big enough to overshadow the extra uterus in which case the ultrasound technician might not see it.