According to the Mayo Clinic, menstrual flow may normally occur as often as every 21 days, and menstrual cycles tend to become shorter with increasing age. That being the case, menstruating twice in a given month is not inherently abnormal.
The length of the menstrual cycle, the interval between menstrual flows, is more meaningful than the number of times in a month that menstruation happens. Relatively long cycles are more common when women first start menstruating, but menstrual cycles tend to become more regular and shorter after the first few years, and if the cycle shortens over time from slightly more than a month to slightly less than a month, then it's very much expected that menstruation will sometimes happen twice in the same month, once early in the month and again late in the month. Oral contraceptives generally make the length of the menstrual cycle much more predictable.
On the other hand, if the interval is much shorter than usual and if the bleeding is otherwise not typical of menstrual bleeding, it's possible that the bleeding is from another cause. The Mayo Clinic recommends seeing your doctor if there is vaginal bleeding between periods or if menstruation happens more frequently than every 21 days.