Tax underpayment. Who caused it?

I started a new job the week after leaving my old job. Received an email this week from HMRC stating I have underpaid tax and as a result my tax code will change until April 2025 to pay back the £209.00 I have apparantly underpaid. I have always worked. I have always updated HMRC whenever necessary. So if anything has been miscalculated, it’s not from me not declaring things correctly. So why should I have to pay this? I can see from forums online that it could be the employer or pension providers mistake but I have no way of knowing this. It just seems so unfair. New job, but my wages are lower because of this.

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Hi rebeccalyon Lyon,
If you were paid by your old employer and new employer in the same month, and both operated the same code, this may have caused the underpayment.
To confirm this, and to help us advise on what you can do, contact us by webchat or phone via Income Tax: general enquiries.
Thank you.