It could be a number of things that will make a woman have two periods in one month. It could be caused by stress and hormone imbalances or even cancer. Women that have two or more periods in one month should see a OBGYN to know what is really going on.
Too frequent periods may be a sign of hormonal dance or imbalance. The incidence is usually high for individuals who are near their menopausal period. Other causes include a defective uterine lining or presence of polyps and cysts.