why i keep getting duplicated lists in my recyclerView?

Question

Can anyone show me what have I done wrong here, I am using recyclerView with livedata every time I launch the app it spawns with a random list and sometimes its full of duplicates. Here is the code that assigns data to the adapter:

adapter = new PaymentsByDateParentAdapter(getContext());
    recyclerView.setAdapter(adapter);

    viewModel = new ViewModelProvider(this).get(SchoolViewModel.class);

    viewModel.getAllPayments().observe(getViewLifecycleOwner(), new Observer<List<Payment>>() {
        @Override
        public void onChanged(List<Payment> payments) {
            for (Payment i : payments) {
                datesWithDupes.add(i.getPaymentDate());
            }
            for (String i : datesWithDupes) {
                if (!datesWithoutDupes.contains(i)) {
                    datesWithoutDupes.add(i);
                }
            }
            for (String date : datesWithoutDupes) {
                viewModel.getPaymentsByDate(date).observe(getViewLifecycleOwner(), new Observer<List<Payment>>() {
                    @Override
                    public void onChanged(List<Payment> payments) {
                        PaymentByDate paymentByDate = new PaymentByDate(date, payments);
                        paymentByDateList.add(paymentByDate);
                        adapter.setPayments(paymentByDateList);
                    }
                });
            }
        }
    });

Answer 1

Clear the arraylists datesWithDupes , paymentByDateList and datesWithoutDupes before the for loops.

datesWithDupes.clear();
datesWithoutDupes.clear();
paymentByDateList.clear();

for (Payment i : payments) {
    datesWithDupes.add(i.getPaymentDate());
}
for (String i : datesWithDupes) {
    if (!datesWithoutDupes.contains(i)) {
        datesWithoutDupes.add(i);
    }
}
for (String date : datesWithoutDupes) {
    viewModel.getPaymentsByDate(date).observe(getViewLifecycleOwner(), new Observer<List<Payment>>() {
        @Override
        public void onChanged(List<Payment> payments) {
            PaymentByDate paymentByDate = new PaymentByDate(date, payments);
            paymentByDateList.add(paymentByDate);
            adapter.setPayments(paymentByDateList);
        }
    });
}