Second letter asking whether I have a TV License... three weeks after I have already told them I don't.

End of January I received a letter asking me to confirm whether I still don't need a license. I completed it online. I received a reference number by email. Then last week another letter came, saying my no-license statement had expired, and that I need to complete the form or risk investigation. I panicked and filled the form again, only to receive a reference number by email...which was the same as the one in the email before!

My questions:

1/ should I be worried that they are resending the same letter even though I already completed the form before?

2/ I noticed the reference number in the letter they sent me, and the one in the acknowledgement email they send afterward, are different : is it normal?