I've already paid the July 31 balancing payment - so why I am I being asked to pay again?

Do I have to make a July 31 payment on account if I have already paid in full in January and owe nothing? I am getting letters telling me to make a return. However, my current position on my online account is that I have 'nothing to pay'.

Hi gdex,
We may need to check your record, to confirm what's being asked for, and why.
Please contact us by webchat or phone here:
Self Assessment: general enquiries
Thank you.