My friend had her period already and 4 days later got it again, why is that and is it normal?

Very common. If this happens often she will need to see her obgyn for an exam and testing. If it is a one time thing then no big deal. Depending on age and fertility plans there are lots of treatment options. Birth control pills can regulate the cycle. Mirena (levonorgestrel) iuds can minimize periods. Lysteda is a great new hormone free drug used to stop heavy periods. If older may even want an endometrial ablation.