Firebase - How to make sure Authentication for an already authenticated user doesn't happen again?

Question

I've been trying to get a fix on this situation for days now, and I can't seem to find a solution. So here is what I want to do: I have an app which uses firebase-ui-auth to authenticate users via email/Google account. As soon as a user is authenticated, I write the user's details to a Firebase database child 'clients'. Now I want to make sure that the next time the same user logs in, his details should not be written to the 'clients'. To do this, I created another child 'reg_clients', to which I push EmailID of the user who has logged in before. Now, as there was no way in Firebase to search on the server side whether currentUser.getEmail() already exists in 'reg_clients', I use:

@Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if(requestCode == RC_SIGN_IN) {
            if (resultCode == RESULT_OK) {
                // user is signed in
                clientCompletedRegistrationReference.addListenerForSingleValueEvent(clientCompletedRegistrationValueEventListener);

                Log.d("in onActivityResult","user is:"+user.getDisplayName());
                Log.d("in onActivityResult","emailAddressList is:"+ Arrays.toString(emailAddressList.toArray()));
                Log.d("in onActivityResult","user.getEmail() is"+ emailAddressList.contains(user.getEmail()));

                if(emailAddressList.contains(user.getEmail())) {
                    Toast.makeText(NavDrawerActivity.this, "You're signed in again", Toast.LENGTH_SHORT).show();
                } else {
                    // Gets photoUrl when Google+ photo is set, otherwise returns null. Appending '?sz=100' at end gets a photo of 100x100 px
                    String photoUrl = user.getPhotoUrl() != null ? user.getPhotoUrl().toString()+"?sz=100": null;
                    Toast.makeText(NavDrawerActivity.this, "You're signed in for the first time", Toast.LENGTH_SHORT).show();

                    Client client = new Client(user.getDisplayName(),0,photoUrl,user.getDisplayName().replaceAll("\\s+","").toLowerCase(),"Brooklyn,NY");
                    // User has just created an account, save his/her details to the 'Clients' child
                    clientDatabaseReference.push().setValue(client);
                    // When user is registering for the first time, pushes his/her email ID to the 'Completed Registration' child
                    clientCompletedRegistrationReference.push().setValue(user.getEmail());
                }
                return;
            }
        }

In the database, there is always (initially)

--reg_clients
 |
 ---K_3uY0-M4ZCkRSt53SZ: "xyz@gmail.com"  

But still the log shows
in onActivityResult user is:XYZ
emailAddressList is:[]
user.getEmail() isfalse

Due to this condition being false the data gets pushed to both the clients and reg_clients as new data, and thus multiple entries of the same user is being made in the database.

And then onDataChange of clientCompletedRegistrationValueEventListener is invoked which populates the emailAddressList, but it has no purpose now, as duplicate entry has already been made.

Please help me in this approach or please let me know if you have a better way to do this. Thanks

Edit: Added code for clientCompletedRegistrationValueEventListener

clientCompletedRegistrationValueEventListener = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                for (DataSnapshot emailSnapshot : dataSnapshot.getChildren()) {
                    Log.d("Adding",emailSnapshot.getValue().toString()+" to emailAddressList");
                    emailAddressList.add(emailSnapshot.getValue().toString());
                    Log.d("in onDataChange","emailAddressList is:"+Arrays.toString(emailAddressList.toArray()));
                }
            }

Answer 1

When you add a listener you don't get the value immediately.You have to wait for the response.So ,Put that code inside onDataChange of the listener.Like this

  clientCompletedRegistrationValueEventListener = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                for (DataSnapshot emailSnapshot : dataSnapshot.getChildren()) {

                    Log.d("Adding", emailSnapshot.getValue().toString() + " to emailAddressList");
                    emailAddressList.add(emailSnapshot.getValue().toString());
                    Log.d("in onDataChange", "emailAddressList is:" + Arrays.toString(emailAddressList.toArray()));
                }
                    Log.d("in onActivityResult","user is:"+user.getDisplayName());
                    Log.d("in onActivityResult","emailAddressList is:"+ Arrays.toString(emailAddressList.toArray()));
                    Log.d("in onActivityResult","user.getEmail() is"+ emailAddressList.contains(user.getEmail()));

                    if(emailAddressList.contains(user.getEmail())) {
                        Toast.makeText(NavDrawerActivity.this, "You're signed in again", Toast.LENGTH_SHORT).show();
                    } else {
                        // Gets photoUrl when Google+ photo is set, otherwise returns null. Appending '?sz=100' at end gets a photo of 100x100 px
                        String photoUrl = user.getPhotoUrl() != null ? user.getPhotoUrl().toString()+"?sz=100": null;
                        Toast.makeText(NavDrawerActivity.this, "You're signed in for the first time", Toast.LENGTH_SHORT).show();

                        Client client = new Client(user.getDisplayName(),0,photoUrl,user.getDisplayName().replaceAll("\\s+","").toLowerCase(),"Brooklyn,NY");
                        // User has just created an account, save his/her details to the 'Clients' child
                        clientDatabaseReference.push().setValue(client);
                        // When user is registering for the first time, pushes his/her email ID to the 'Completed Registration' child
                        clientCompletedRegistrationReference.push().setValue(user.getEmail());
                    }

                }
            };