remove duplicate xml elements from doc builder list in java

Here's what I'm trying to do:

• load in an XML file, then scan the XML file to put all the elements in a combobox so that a user can select one.

I've got this working up to a point in the fact that there are several elements with the same tags within the body of the XML file itself, and thus, the element occurs more than once in the drop down list, is there a way in my for loop to compare what's already there and remove duplicates?

Here's the entire method I've got

     public static void readXML(String filePath) {

        try {

            //Gets selected XML file
            File XmlFile = new File(filePath);
            DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
            DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
            Document doc = dBuilder.parse(XmlFile);

            //Searches all text
            doc.getDocumentElement().normalize();

            //Make a non-editable combo box
            JComboBox<String> comboBox = new JComboBox<String>();
            comboBox.setEditable(false);

            //Get all the XML elements from the file
            NodeList list = doc.getElementsByTagName("*");

            //TODO:
            //Make sure all XML elements only appear once in the list

            //Populate combobox with all elements from input file
            for (int i = 0; i < list.getLength(); i++) {

                Element element = (Element)list.item(i);                
                String item = element.getNodeName().toString();
                //Add comparison here??
                comboBox.addItem(item);

            }

            //Add Combo box and refresh the frame window so that it appears
            buttonPanel.add(comboBox);
            frame.revalidate();

            //Add action listener show which XML element has been selected
            comboBox.addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent event) {

                    //Get the source of the component, which is the combo box
                    JComboBox<?> comboBox = (JComboBox<?>) event.getSource();

                    //Print the selected item
                    String selected = comboBox.getSelectedItem().toString();
                    log.append("The selected XML element is: " + selected + newline);
                }
            });


        } catch (Exception e) {
            e.printStackTrace();
        }
    }

EDIT:

A second problem I faced was to make sure that all the elements were in alphabetical order. I fixed this issue by doing the following:

// Make a sublist so that the elements can be sorted
List<String> subList = allValues.subList(0, allValues.size());
Collections.sort(subList);

// Add the items from the subList to the comboBox
for (int j = 0; j < subList.size(); j++) {
String listItem = subList.get(j).toString();
comboBox.addItem(listItem);
}