If you've purchased land and discovered the seller has since sold the same plot to another buyer, you have legal recourse. As the first buyer with documentation, you can take action. While a criminal complaint against the seller is an option if they are alive, if they have passed away, you can send a legal notice to the subsequent buyer and the seller's legal representatives. This notice should assert your ownership. Based on their responses, you may need to file a suit for declaration and injunction with the court to secure your rights to the land.