The website you bought was an asset of the previous owner (business?).
The invoice ($5000 double payment) would have been made out to the previous owner, as you dont own the previous owner, you arent liable.
All you have done is bought an asset from a business, it’s always a good idea to have a generic contract which you can easily fill in and flick across – especially when it comes to taking control of business names, businesses, and accounts and what you are actually purchasing.