This is an issue that probably doesn't come up enough for pre-set responses.
Do you mean the seller (may be the same as the manufacturer?) has contacted you with this information and request to pay again?
In your position, I would proceed with caution and would take these steps.
Message the seller just once more. Tell them you have paid and cannot be expected to pay again. Tell them they can either cancel the transaction and return your payment or send the item. (Whatever their response, i would not message them again.)
Then, if the item does not arrive by the latest estimated delivery date (see your email confirming payment), open a case for 'item not received' using the Money Back Guarantee.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)