already vaccinated cat, vaccinated again 3 months later
No. 1 -- Don't pay for the vaccines. If you paid, hopefully it was by credit card.
No. 2 -- Don't use them again. They're either incompetent or don't give a crap.
No. 3 -- The extra doses of vaccine won't hurt her. And cases of the sarcomas are still rare. Just keep an eye on her, but chances are she will be just fine.
Thanks for your question and I'm sorry that this has happened to your kitten.
In regards to injection site sarcomas, thankfully these are quite rare. I've only seen one case in the last 8 years and that was in an older cat who had repeatedly had injections with adjuvants in their neck over a long period of time. The chances of them are between 1 in 1,000 to 1 in 10,000. Considering your kitten has only had one vaccine with an adjuvant I wouldn't be overly worried. Especially as you're choosing vets that have vaccines without adjuvants, it's likely that your kitten won't have one again. There's some more information about this here; Feline Injection Site Sarcomas: Causes, Symptoms & Treatment - Cats.com
In terms of your kitten receiving an additional vaccine that they didn't need, I wouldn't be concerned about this. Kittens and cats often get extra vaccines if they've missed their booster shot or if they've been found/rehomed and their vaccination history was unknown. You can chat to your vet about tailoring a vaccine protocol based on your kitten's lifestyle, this avoids over-vaccination.
I can understand why you're frustrated about the veterinary visit. I think it's worth contacting the vets and explain that you hadn't asked for her to be vaccinated (especially with an adjuvanted vaccine). Having a discussion with them might shed some light on the situation and hopefully resolve things.
I hope that helps,