Incompetecy: Cylinder no delivery then double delivery

We ordered an oxygen cylinder and were promised delivery 3 days later. Nothing came. On phoning office they knew of no order. We ordered again. Nothing came. After many calls and now 8 days without oxygen we started a big complaint. No operations or emergencies needing O2 could be done. So finally a delivery took place. Shockingly, 4 days later they deliver a 2nd cylinder while removing the previous still full and unused tank without checking if the cylinder is empty or noticing its heavy weight while full. My staff present was different from the previous delivery and did not know that the tank had already been replaced. We notified the office immediately of the erroneous double delivery. After multiple mails and 2 months later (?) Mrs Mercy Mashakane confirmed that they will credit me for the 2nd delivery. Surprise: a month later I receive a bill again for both cylinders and no credit. On enquiering with Mrs Mercy which is unreachable via phone, the mail reply now was that we should not have allowed the driver to unload the 2nd cylinder. So why first telling me credit will be given? This is pathetic poor customer service and incompetency at the highest level which eventually will ruin a business even as big as Afrox.