Duplicate entry for primary key - registration form

I have one simple registraton form with jsp, servlet and mysql. But, I can register only one user, then i got this error:

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 ...

This is also my method for insert in db:

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;
}

Thank you.

You are always setting the value of your id column to 1 via preparedStatement.setInt(1, 1). Since you say you have an auto-increment on the id column, remove id from your SQL, remove the aforementioned line and adjust the column indexes (first arguments) for all the other setString() method calls.

With auto increment set you don’t have to specify the ‘id’ column in your sql insert statements.

Here’s some information on the auto increment from MySQL’s website: https://dev.mysql.com/doc/refman/8.0/en/example-auto-increment.html

From your code it looks like you are inserting the value 1 always for the id column, try implementing the autoGenerate properly. For more help, you can have a look on similar implementation