"How do i proceed?" You answered your question "Should i go to the resolution center and press make full refund?" << Do this! "If i do that do i need to add again the same amount of money so i can make the refund?" No since you added it once you wouldn't need to add it again. PayPal paid the buyer back on your behalf and now that you added funds you paid PayPal back. That's all there is to it. "That would mean i have been making double payments." Correct, hence why you only need to do it once, which you already have. Good Luck!
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