SET SQL_SAFE_UPDATES = 0;

public class DatabaseWork {
// подключение к бд
private static final String url = "jdbc:mysql://localhost:3306/players?autoReconnect=true&useSSL=false";
private static final String user = "user";
private static final String password = "password";

// переменные подключения
private static Connection con = null;
private static Statement stmt = null;
private static ResultSet rs = null;

// строка запроса
private String query;

public void banIp(String ipaddress, String reason) {
    query = "INSERT INTO `bannedip` (`ip`, `reason`) VALUES ('" + ipaddress + "', '" + reason + "');";

    try {
        con = DriverManager.getConnection(url, user, password);

        // getting Statement object to execute query
        stmt = con.createStatement();

        stmt.executeUpdate(query);
    } catch (SQLException e) {
        e.printStackTrace();
    }
    finally {
        try {
            con.close();
        } catch (SQLException se) {
            se.printStackTrace(); }
        try {
            stmt.close();
        } catch (SQLException se) {
            se.printStackTrace(); }
    }
}

DatabaseWork db = new DatabaseWork();
db.banIp("hjg996k", "reason");

com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Duplicate entry 'hjg996k' for key 2