You can have two periods in a one calendar month if you start on the 1st or 2nd day of the month. You could begin the next cycle at the end of the same month. Having a period twice in the same cycle is not as normal. It could be due to hormonal changes or a thickening of the uterine wall. Polyps and fibroids in the uterine cavity can also cause irregular periods.
If you are young and just starting puberty then two periods a month could be normal because your body is trying to adjust to the changes. For others two periods a month is not normal and could be a sign of something more serious. You should talk to your doctor about this problem.