We've been inundated this year already with overpayment notices from HMRC. This year they seem to have been issuing notices very early.
The main cause seems to be that they have been issued with dates before the 19th April, which is of course the due date for your March PAYE/NI.
So in most cases this is a timing difference and causes a lot of unnecessary work. We've already made representations to HMRC to hold any notices being issued until after the 19th April in future years.