How would I print values from a HashMap while not printing duplicates?

Question

I'm trying to fix this piece of code where I'm printing from a hashmap having a list of plate numbers and owners (that format). I'm trying to print out just the owners via printOwners(); but I can't get it to not print duplicates.

I've played around with it for a while, just can't seem to skip over duplicates.

Here is my code:

import java.util.ArrayList;
import java.util.HashMap;

public class VehicleRegister {

    private HashMap<RegistrationPlate, String> owners;

    public VehicleRegister() {
        owners = new HashMap<RegistrationPlate, String>();
    }

    public boolean add(RegistrationPlate plate, String owner) {
        //search for existing plate
        if (!(owners.containsKey(plate))) { // add if no plate
            owners.put(plate, owner);
            return true;
        }

        //if plate is found, check for owner
        else if (owners.keySet().equals(owner)) {
           return false;
        }

        return false;
    }

    public String get(RegistrationPlate plate) {
        return owners.get(plate);
    }

    public boolean delete(RegistrationPlate plate) {
        if (owners.containsKey(plate)) {
            owners.remove(plate);
            return true;
        }

        return false; 
    }

    public void printRegistrationPlates() {
        for (RegistrationPlate item : owners.keySet()) {
            System.out.println(item);
        }
    }

    public void printOwners() {

        for (RegistrationPlate item : owners.keySet()) {
            System.out.println(owners.get(item));            
        }
    }
}

Answer 1

To remove the duplicates, use a HashSet<String> :

public void printOwners() {
    for (String s : new HashSet<>(owners.values())) {
        System.out.println(s);            
    }
}

Alternatively with Java 8 Stream and the distinct() method:

public void printOwners() {
    owners.values().stream().distinct().forEach(System.out::println);
}