主键重复输入-注册表

我有一个简单的注册表格,包含jsp,servlet和mysql。但是,我只能注册一个用户,然后出现此错误:

Severe: Message: Duplicate entry '1' for key 'PRIMARY'. Of course, when I delete that user from database I can create again but just one user. Also, I put auto increment on that column:

CREATE TABLE user (id int(3) NOT NULL AUTO_INCREMENT ...

这也是我在db中插入的方法:

public class UserDAO {

public int registerUser(User User) throws ClassNotFoundException, SQLException {
    String INSERT_USERS_SQL = "INSERT INTO user"
            + "  (id, firstName, lastName, username, password, email) VALUES "
            + " (?, ?, ?, ?, ?, ?)";

    int result = 0;

    Class.forName("com.mysql.jdbc.Driver");

    try (Connection connection = DriverManager
            .getConnection("jdbc:mysql://localhost:3306/cs230projekat","root","");
            // Step 2:Create a statement using connection object
            PreparedStatement preparedStatement = connection.prepareStatement(INSERT_USERS_SQL)) {
        preparedStatement.setInt(1, 1);
        preparedStatement.setString(2, User.getFirstName());
        preparedStatement.setString(3, User.getLastName());
        preparedStatement.setString(4, User.getUsername());
        preparedStatement.setString(5, User.getPassword());
        preparedStatement.setString(6, User.getEmail());

        System.out.println(preparedStatement);
        // Step 3: Execute the query or update query
        result = preparedStatement.executeUpdate();

    } catch (SQLException e) {
        // process sql exception
        printSQLException(e);
    }
    return result;
}

谢谢。