My period comes twice this month (18 days apart). Is this normal?

It can happen. Every woman experiences an off period at least once in her life.

It could be for many reasons, diet change, medication, weight loss, weight gain, etc. etc.

If you're concerned then talk to your doctor.

Good luck.

How old are you? If you're in those peri-menoupausal years, it's very common.

How long has this been happening? If it's been for a while, some women ovulate from each ovary each month, just at different times, causing two periods in one month.