Is That Suppose To Happen?

So I recently opened my very first bank account and I ordered something by mistake online and contacted the seller to cancel the order so I could get my money back and they said okay .but I check my account balance today and I see where a transaction was made for the same order I requested to be cancelled but I got an email saying that they processed my refund so why is the same amount of money I paid for the item I wanted cancelled being taken off my card twice ? I'm only 18 and this is my first bank account so I don't know if this is suppose to happen