Toast message inside MutableLiveData.observe() working repeatedly

I have a register form and ı want to show a message to the user based on signed up successfully or not.I store that message in my view model which has a mutable live data object to store that message and observing that object from my activity.My problem is when ı click to the register button first time it works normally,showing Toast with my message as expected but when ı click that button again Toast message show my message two times.I really didn't understand what's wrong with my code.Here is my code.

RegisterActivity

  mViewModel = new ViewModelProvider(this, ViewModelProvider.AndroidViewModelFactory.getInstance(getApplication())).get(RegisterActivityViewModel.class);
mRegisterBtn.setOnClickListener(v -> {
register();
}); 
private void register() {
if (!GenericMethods.checkInputField(mKadi) || !GenericMethods.checkInputField(mEmail)
|| !GenericMethods.checkInputField(mPAss)) {
Toast.makeText(this, "Alanlar boş geçilemez", Toast.LENGTH_SHORT).show();
return;
}
signup();
}
private void signup(){
kAdi = mKadi.getText().toString().trim();
email = mEmail.getText().toString().trim();
parola = mPAss.getText().toString().trim();
parolaTekrari = mValidpass.getText().toString().trim();
il=mIlTv.getText().toString().trim();
ilce=mIlceTv.getText().toString().trim();
getRadioValue(mRadioGroup);
üniversite=mUniTv.getText().toString().trim();
User user = new User(kAdi,email,parola,cinsiyet,il,ilce,üniversite);
mViewModel.signup(user);
mViewModel.signupData.observe(this,status ->{
Toast.makeText(RegisterActivity.this, status, Toast.LENGTH_SHORT).show();
});
}

RegisterActivityViewModel

public MutableLiveData<String> signupData = new MutableLiveData<>();
private String statusString=null;
public void fetchFromRemote() {
isLoading.setValue(true);
disposable.add(RetroService.getInstance().getIller()
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribeWith(new DisposableSingleObserver<List<Response>>() {
@Override
public void onSuccess(@NonNull List<Response> responses) {
isLoading.setValue(false);
adresList.setValue(responses);
}
@Override
public void onError(@NonNull Throwable e) {
isLoading.setValue(false);
}
}));
}
public void signup(User user) {
Repo.getInstance().signup(user)
.enqueue(new Callback<Void>() {
@Override
public void onResponse(Call<Void> call, retrofit2.Response<Void> response) {
if (response.code() == 200) {
statusString="Success!";
} else if (response.code() == 400) {
statusString="email already exists";
}
signupData.setValue(statusString);
}
@Override
public void onFailure(Call<Void> call, Throwable t) {
signupData.setValue(t.getMessage());
}
});
}
@Override
protected void onCleared() {
super.onCleared();
disposable.clear();
}

Because the "register" function creates a new observer everytime it gets called. This is the observer:

Observer<String> signupDataObserver = status -> {
Toast.makeText(RegisterActivity.this, status, Toast.LENGTH_SHORT).show();
}

Everytime the button get clicked the function add a new observer on the MutableLiveData "signUpData":

mViewModel.signupData.observe(this, signupDataObserver);

So if you move the "observe" function into the "OnCreateView" event the problem should be solved, because the fragment will register only 1 observer when the view gets created and not when the button gets clicked. Another option would be to delete the observer after the toast message has been delivered:

Observer<String> signupDataObserver = status -> {
Toast.makeText(RegisterActivity.this, status, Toast.LENGTH_SHORT).show();
mViewModel.signupData.removeObserver(signupDataObserver);
}
mViewModel.signupData.observe(this, signupDataObserver);