Tax Record - Missing A Previous Employer

I got a tax refund in the post today - YAY Was a bit suspicious given that I'd already had one a few weeks back, so went online to check. This new one was showing, the previous one (which I'd cashed) was not. Strange. Then I notice that there is no record of my previous employer (18 months between November 2020 and May 2022) on there, so no PAYE or NI contributions showing for that period (which straddles three tax years). It's not like their a dodgy employer or anything - 500+ employees and one of the largest in their field in the UK. I still have access to the previous employer's payslip portal, so went on and downloaded all of my payslips and P60s - I was taxed and NI'ed every month bar emergency tax on the first one, which was seemingly corrected by overtax in the second. No typos in NI number or anything like that. I can only theorise that for some reason I must have two tax records with HMRC, this employer is on the second and the first tax refund was said record thinking I'd only worked 2 months in FY22-23 (and hence it was refunding me the tax I'd paid in the two months of that year) I can't quite figure it out and I'm not sure what to do - any advice? Obviously call up HMRC when time allows but I'm terrified of suddenly being given an 18 month bill or something despite having the evidence to hand.

Hi Arkturus,

Before you cash the second payble order, you should contact HMRC, to ensure this is not a mistake.
If you complete a self assessment tax return, you can webchat atSelf Assessment: general enquiries (out telephone lines are closed until 4 September).  
If you do not complete tax returns, you can phone the Pay as you earn helpline at 0300 200 3300, text relay 18001 0300 200 3300 or from overseas +44 13 5535 9022 (Monday to Friday: 8am to 6pm Closed on Saturdays, Sundays and Bank Holidays).

Thank you.