Authorisation hold excuse for double charges

Transactions during May 2020 and June 2020. The tip was charged with my food order bill and then a separate charge came off for the tip portion again. I sent my receipt and the transactions on my bank statement. When querying this and asking for a refund I get this response almost every time:

"After looking into your account, we can confirm that only one charge has processed for this order.

However, you may see a temporary authorisation hold appearing on your bank account as a pending transaction. This usually releases within a few hours but time frames may vary depending on your bank's policy."

This is months ago and the money went off. No authorisation hold. They refuse to refund me and also refuse to phone me to discuss the issue