Answered question
Last monday, it has already passed 9days. I asked to CS center about duplicated payment for same add-on.
Whenever I ask about that issue. They always answer they still work on it without the answer I asked.
How long do I have to wait?
Fizz created a ticket. They will answer you.
In the meantime, if you follow-up with them, you can ask if they need more information or proof in order to issue the refund.
Other answers
Yes using the facebook chat
I got an error and they rembourse me right away. But I used the chat bubble. Probably I was lucky
I waited for two days using Facebook.But I didn't have payments issues.