O2 could not debit, I transfered, but now it's on my invoice to be paid again

Did your transfer maybe reach o2 after the new invoice was issued?

I doubt, because I transfered on the 15.07 and the bill’s date in 19.07. It has been time enough to have received the transfer… :/

No, the 15th was Friday, so your account will only have been credited yesterday or today. The new invoice will not have taken this into account.

Therefore, how can this be solved?
It was already paid, but the invoice accounts otherwise. Will only this month amount be debited from my account, then?

As long as the money is in your customer account before the direct debit is processed (very likely), only the new amount will be debited.

Hello @Salerno.l ,

I checked your account with us and we have received your payment, as such only the regular monthly fee + the additional 5,20€ will be debited from you. As bs0 surmised the date of your payment meant that it couldn’t be reflected on the bill that was issued to you on the 19th. I hope this makes everything clearer for you, if you have more questions, let us know.

Kind regards, Sven