To include a note with the form?

[StressedD]

In July, I was caught by a speed camera. I received the request for driver information, filled the form in and sent it back the same day it arrived.
Since then, I've been waiting to find out the outcome, but heard nothing until Saturday when another letter arrived. This one is a final request for driver information.
I've tried phoning the number on the letter to ask about whether, if I send it back, should they find the original, I will end up with 2 fines for the same offence, but the phone line is either engaged or they hang up immediately. I've also tried emailing, with no response.
Would it be unreasonable to include a note with the form explaining that, I had returned the form in July?

Not unreasonable. Can you send this one recorded delivery?

I would write on the actual form, 'please note this is a duplicate, I completed and returned this form in July 2020 regarding the same incident'. If you do a note alongside the paperwork it will probably get lost!

Thanks. I've put an asterisk at the bottom of the form and explained there. There really wasn't much room on the form. Hopefully, this one will be received by the police/not lost in a RM black hole.