How do l add a node to the root node without overriding any existing nodes in XML

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JavaFX

I have an xml file with a root node called Assignments and if l add a an child element called Assignment it successfully adds with all properties date,course etc....the problem arises if l try adding another assignment instead of adding it overrides the previously inserted data

What I have tried:

try
{
File assigns = new File("xmlfiles/Assignments.xml");
DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
Document doc = dBuilder.parse(assigns);

Node rootNode = doc.getDocumentElement();
Element assign = doc.createElement("Assignment");
rootNode.appendChild(assign);

Element modulename = doc.createElement("ModuleName");
modulename.appendChild(doc.createTextNode(txtModule.getText()));
assign.appendChild(modulename);

LocalDate ld = datePicker.getValue();
Element date = doc.createElement("Date");
date.appendChild(doc.createTextNode(ld.toString()));
assign.appendChild(date);

Element somenotes = doc.createElement("Notes");
somenotes.appendChild(doc.createTextNode(txtNotes.getText()));
assign.appendChild(somenotes);

TransformerFactory transformerFactory = TransformerFactory.newInstance();
Transformer transformer = transformerFactory.newTransformer();
DOMSource source = new DOMSource(doc);
StreamResult result = new StreamResult(assigns);
transformer.transform(source, result);

}
catch(Exception ex)
{
JOptionPane.showMessageDialog(null, ex);
}