Is this normal??!

Every 3 months I have 2 periods in that month. So for example in January I have a period in the middle of the month then febuary I have one at the beginning and one at the end and then in March I have one in the middle again and this happens every 3 months so carrying on with the example my next double period would be in may. When I have my double periods it usually makes up about 9-12 days of period in just one month. Is this normal or is it something I should be worried about?