Objects are created without intention

I am trying to create a LinkedList and then create a menu asking to create a Teacher, Show Teacher list.
The thing is that I can create the Teacher object and add it to the LinkedList, but, when I select the Second option, Show Teacher List more than one time without creating a new Teacher, the programs continue to creating objects.

Here is my code:

    import java.util.*;
import javax.swing.JOptionPane;
public class Prueba {
public static void main(String[] args) {
LinkedList<Teacher> teacher = new LinkedList();
int id, salary,op;
String name, programm, teacherList ="";
Teacher t1 = new Teacher();
op=Integer.parseInt(JOptionPane.showInputDialog("Please, pick an option: \n"
+ "1. Add Teacher. \n"
+ "2. Get Teacher List. \n"
));
while(op!=0){
if(op == 1){
id=Integer.parseInt(JOptionPane.showInputDialog("Ingrese ID del docente: "));
name=JOptionPane.showInputDialog("Ingrese nombre del docente: ");
programm=JOptionPane.showInputDialog("Ingrese programa del docente: ");
salary=Integer.parseInt(JOptionPane.showInputDialog("Ingrese salario del docente: "));
Teacher d = new Teacher(id,name,programm,salary);
teacher.add(d);
}else if(op == 2){
Iterator p = teacher.iterator();
while(p.hasNext()){
teacherList = teacherList + "\n"+ p.next();
}
JOptionPane.showMessageDialog(null, teacherList);
}
op=Integer.parseInt(JOptionPane.showInputDialog("Please, pick an option: \n"
+ "1. Add Teacher. \n"
+ "2. Get Teacher List. \n"
));
}
}

}

Class Teacher:

public class Teacher {
private int id;
private String name;
private String programm;
private int salary;
public Teacher() {
}
public Teacher(int id, String name, String programm, int salary) {
this.id = id;
this.name= name;
this.programm = programm;
this.salary = salary;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name= name;
}
public String getProgramm() {
return programm;
}
public void setProgramm(String programm) {
this.programm = programm;
}
public int getSalary() {
return salary;
}
public void setSalary(int salary) {
this.salary = salary;
}
@Override
public String toString(){
String message;
message = "Id: "+this.id+" - Name: "+this.name+" - Programm: "+this.programm+" - Salary: $"+this.salary;
return message;
}

}

While navigating the list, you need to add the string representation of each teacher (toString()) to a string and display the string once you have iterated the whole list.

Do it as follows:

public class Prueba {
public static void main(String[] args) {
LinkedList<Teacher> teachers = new LinkedList();
int id, salary, op;
String name, programm, teacherList = "";
Teacher teacher;
op = Integer.parseInt(JOptionPane
.showInputDialog("Please, pick an option: \n" + "1. Add Teacher. \n" + "2. Get Teacher List. \n"));
while (op != 0) {
if (op == 1) {
id = Integer.parseInt(JOptionPane.showInputDialog("Ingrese ID del docente: "));
name = JOptionPane.showInputDialog("Ingrese nombre del docente: ");
programm = JOptionPane.showInputDialog("Ingrese programa del docente: ");
salary = Integer.parseInt(JOptionPane.showInputDialog("Ingrese salario del docente: "));
teacher = new Teacher(id, name, programm, salary);
teachers.add(teacher);
} else if (op == 2) {
StringBuilder sb = new StringBuilder();
for (Teacher t : teachers) {
sb.append(t.toString()).append(System.lineSeparator());
}
JOptionPane.showMessageDialog(null, sb.toString());
}
op = Integer.parseInt(JOptionPane
.showInputDialog("Please, pick an option: \n" + "1. Add Teacher. \n" + "2. Get Teacher List. \n"));
}
}
}

Notes:

1. I've used a StringBuilder variable instead of a String variable to collect the string representation of each teacher to avoid too many strings getting created. If you wish to use a String variable for it, you can do so.
2. I've used enhanced for loop to navigate the list (teachers). If you wish to use Iterator for it, you can do so.

A sample output:

try add teacherList = "" after JOptionPane.showMessageDialog(null, teacherList);

The problem isn't that the objects are duplicated, it's that you are adding the new text to the teacherList variable without clearing it.
So after (or before)

while(p.hasNext()){
teacherList = teacherList + "\n"+ p.next();
}
JOptionPane.showMessageDialog(null, teacherList);

You should probably add something like

teacherList = "";