A few different conditions or life changes can cause female patients to menstruate twice a month, as well as simply just having a shorter menstrual cycle than the average period haver.
If having more than one period is not a normal occurrence for the patient and persists for cycles that follow, the cause of this could be the onset of menopause (or perimenopause), which can cause the heavier, more frequent menses as well as the opposite in lighter, less frequent periods. The patient being pregnant could also manifest as the patient having two periods within the same month as the patient may experience spotting, which can be normal but should be monitored and its frequency reported to the patient's healthcare provider. The other side of this coin in miscarriage can unfortunately lead to the appearance of a second period, as the patient is expelling the embryo. Irregular periods pertaining to frequency could be a manifestation of polycystic ovary syndrome (PCOS), a hormone condition in which fluid-filled cyst sacs with immature follicles do not release their eggs (anovulation); its etymology is currently unknown. In the same hormonal realm, thyroid conditions such as hypo- and hyperthyroidism could lead to more frequent menstruation. The presence of uterine fibroids or cysts could lead to more than one period per month, an a predisposition may increase the risk of such occurrence if there is a family history of either. If the patient is taking a new hormonal birth control or intrauterine device (IUD) as a form of contraceptive, there is a possibility that it alters their period schedule and causes a second period. If the patient recently experienced an extreme weight gain or weight loss, their body may respond with a change to their menstruation cycle. Bleeding disorders could cause another period for the patient, hemorrhaging that could be fatal and needs immediately addressing. Lastly, some sexually transmitted infections (STIs) or yeast infections such as chlamydia can cause vaginal irritation leading to spotting and the appearance of a second period.
If the patient is concerned by more than one menstruation cycle in a month, they should contact their healthcare provider and potentially a OB/GYN specialist in order to seek understanding into this event.