What is the ruling on someone who sold a plot of land, not knowing that it had been sold to several people by the first seller, and then sold it to one of their relatives? What should they do?

The sale is void and invalid because it involves selling what one does not own. The Prophet, peace and blessings be upon him, said: "It is not lawful to combine a sale and a loan, nor two conditions in a sale, nor to sell what you do not possess." There is no sin upon you for your ignorance of the prior sale of the land, and the land remains the property of the first buyer.