What would you do if doctor's office keeps double billing you?

I have colon cancer and liver cancer. I get charged for office visit and office in-house bloodwork billed all under my oncologists name each time I'm seen. They break up $20 copy to 8.55 and 11.44 for different services they charge per visit, and keep double billing me 11.44. They make me pay $20 copay on the day of service at the office. I get doubled billed every time Im seen for past 3 years. Each time when double billed, I email them the scanned copy of EOB, $20 copay payment receipt, and the double billed statement and until now I was able to resolve them after multiple attempts of reaching out to both the billing company, and oncologists office. This time they are ignoring my email with EOB, receipt, and double billed statement. I plan on calling the oncologist office and ask for practice manager next week to explain the problem yet again, and to ask they credit 11.44 to my account. What should I do? The amount is fairly small, but I should not have to pay twice, and it is becoming very annoying to have to explain and prove my payments again and again. I noticed cancer is very common, and the oncologist offices get more than enough business, and so it doesn't hurt them to lose some patients. Perhaps, refuse to pay the $20 copay on the day of service, and pay when they bill you? I want to be discretionary and exhaust diplomatic options before finding another oncologist practice. Most oncogist practices are consolidated into big groups in my area. Going someplace else can also be troublesome.