Contract has expired and I’ve cancelled but I’m still debited

I had a WiFi router with data contract which lapsed or expired in Nov, so I cancelled the contract online only to get debited. I called them to rectify the problem. Yesterday 13 Dec I was debited twice the amount. I wanted to take up a new data contract with them but I have had it!!!! This was my 1st contract with them but I already wouldn’t recommend to take a contract up with them. It’s a problem if they debit you unauthorized.