Check with your bank and ensure that the payment went through. Notify them that you are being asked to pay again. If your bank confirms that the payment went through, give the details to the person/company from whom you bought the item and tell them you have paid and do not intend paying again. If you are in the UK, check with the Trading Standards office for the area in which the company is based to see whether anything is known about them. Also check on the internet to see if anyone else has posted something about them.
If your bank says you have paid, then you have so stick to your guns.