Why do I have two payslips with the same week/month number?

The week or month number is determined from the pay date.

If you pay your employees on the same day of each week, the week number will be different every week. If, for example, you pay them on Saturday one week and Friday the next week, these may have the same week number.

With monthly paid employees, each tax month starts on the 6th. If, for example, you pay someone on the 6th April and then on the 5th May, these payslips will both be in month 1.

The week/month number can affect tax but not national insurance. If you have two payslips with the same month number, you will normally find that the second payslip has a higher than usual tax deduction.