Afrox customer care terrible

I decided to take up TIG welding and rented a cylinder 17kg from Afrox in 2014, due to various reasons I only managed to empty the cylinder in 2023 and took it and exchanged it for another cylinder. Since then Afrox started charging me for two cylinders rent because they claim I brought in another customer's(third party cylinder), therefor according to them, I still have my old cylinder and now the new cylinder as well. When I complain they keep on sending me messages that the issue have been resolved. It is impossible for me to exchange or confuse cylinders because I only rented one cylinder ever. I do not have contact with any welders or industries, the cylinders was in my garage at home the whole time. I have been saddled with a cylinder I will never be able to return. The only recourse seems to be a legal one