You were likely not billed for December, hence being billed for both December and January at the beginning of January. It's not really all that uncommon.
Unless you were billed for December and billed twice again in January.
I pay with my credit card, so it went off instantly when I signed up and went off again on the first. In other words, I pay in advance. Point being that you, from now, do too.
I don't like this. Can I pay at the end of the month via cash deposit rather?