Is there a way for only getting a single value for a setOnItemClickListener instead of getting multiple values if an option is clicked multiple times?

I have a checklist and I want to send all the checked items from the list to a Firebase database. I have it sending to the database but there is one problem. If the item is selected, deselected and selected again, the value will be sent twice to the database.

    listView.setAdapter(adapter);
    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
    @Override
    public void onItemClick(AdapterView<?> adapterView, View view, int i, 
    long l) 
    {
       String selectedItem = ((TextView) view).getText().toString();
       selectedItems.add(selectedItem);

       if (unselectedItems.contains(selectedItem)) {
       unselectedItems.remove(selectedItem);
    }
    }
    });

    public void showSelectedItems(View view) {

    FirebaseUser user = mAuth.getCurrentUser();
    String presentStudents = "";

    for (int i = 0; i < selectedItems.size(); i++) {
        dbPresentRef.child(selectedItems.get(i)).push().setValue("Present");
        presentStudents += "-" + selectedItems.get(i) + "\n";
    }
    }

Just add an if condition before adding selectedItem ti your selectedItems list like this.

listView.setAdapter(adapter);
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int i, 
long l) 
{
   String selectedItem = ((TextView) view).getText().toString();
   if(!selectedItems.contains(selectedItem)){
    selectedItems.add(selectedItem); 
   }


   if (unselectedItems.contains(selectedItem)) {
   unselectedItems.remove(selectedItem);
}
}
});

public void showSelectedItems(View view) {

FirebaseUser user = mAuth.getCurrentUser();
String presentStudents = "";

for (int i = 0; i < selectedItems.size(); i++) {
    dbPresentRef.child(selectedItems.get(i)).push().setValue("Present");
    presentStudents += "-" + selectedItems.get(i) + "\n";
}
}