public class Test {
public static void main(String[] args) throws InterruptedException {
    new Thread(new Thready("fuckthisshit")).start();
    new Thread(new Thready("fuckthisshit")).start();
    new Thread(new Thready("fuckthisshit")).start();
    new Thread(new Thready("fuckthisshit")).start();

}

static class Thready implements Runnable {
    private String name;

    public Thready(String name) {
        this.name = name;
    }

    @Override
    public void run() {
        try {
            Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/fp_db", "root", "root");
            connection.setAutoCommit(false);
            connection.setTransactionIsolation(Connection.TRANSACTION_READ_UNCOMMITTED);
            PreparedStatement preparedStatement = connection.prepareStatement("SELECT * FROM users WHERE email =?");
            preparedStatement.setString(1, name);
            if (!preparedStatement.executeQuery().next()) {
                PreparedStatement preparedStatement1 = connection.prepareStatement("INSERT INTO users (email) VALUES (?)");
                preparedStatement1.setString(1, name);
                preparedStatement1.executeUpdate();
                preparedStatement1.close();
            }else {
                connection.rollback();
            }
            connection.commit();
            preparedStatement.close();
            connection.close();
        } catch (SQLException e) {
            e.printStackTrace();

        }
    }
}

static class Thready implements Runnable {
    private static final String JDBC_URL = "jdbc:mysql://localhost:3306/fp_db";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "root";

    private String name;

    public Thready(String name) {
        this.name = name;
    }

    @Override
    public void run() {
        try (Connection conn = DriverManager.getConnection(JDBC_URL, USERNAME, PASSWORD)) {
           conn.setAutoCommit(false);

           if (!isUserAlreadyExist(name, conn)) {
               insertUser(name, conn);
           }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    private boolean isUserAlreadyExist(String email, Connection conn) throws SQLException {
        try (PreparedStatement stmt = conn.prepareStatement("SELECT * FROM users WHERE email = ?")) {
            stmt.setString(1, email);

            return stmt.executeQuery().next();
        }
    }

    private void insertUser(String email, Connection conn) throws SQLException {
        try (PreparedStatement stmt = conn.prepareStatement("INSERT INTO users (email) VALUES (?)")) {
            stmt.setString(1, email);
            stmt.execute();
        } catch (SQLException e) {
            conn.rollback();
        }
    }
}