Yes, the identical set sold yesterday.
The item which sold in July, was this a collection item and did the buyer pay in cash when this was still a possibility to do so ?
If you did not finalise the transaction by stating Item collected and paid, or marked it as dispatched, this is also eBay's wording for a collected item, it would seem then, the item just auto relisted, as it looked like it was still for sale.
You need to send this current buyer a message explain the error, and hopefully they'll understand so you can cancel, citing Buyer Requests to Cancel. This will refund you your seller fee and your seller performance will not be affected.
@samire61