Why do a woman how menstrual twice a month

A woman's cycle is normally every 28 days, not twice a month.

Most women will have a menstrual cycle on a fairly regular basis once a month. Unusual bleeding can happen for a number of reasons and should always be looked into by a woman's doctor. Often there is no real concern, and it usually is just a one off, but sometimes more testing and possible treatment become necessary depending on the cause for the abnormal bleeding.