In August 2017 I worked at a retail company on minimum wage then I got a high salary paying job during the same month so my income overlapped. HMRC was aware but they didn't tax me until after April 2018. Come April 2019 my tax code has changed to a low personal allowance.
So basically I'm paying the tax underpayment through my tax code.
In all honesty HMRC was aware but didn't tax me till months later and I'm not self-employed so I really hope that this is not considered bad character or something.

