MyService.java

import android.app.NotificationManager;
import android.app.PendingIntent;
import android.app.Service;
import android.content.Intent;
import android.graphics.BitmapFactory;
import android.media.RingtoneManager;
import android.os.IBinder;
import android.support.v4.app.NotificationCompat;
import android.util.Log;

import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

public class MyService extends Service {

    private FirebaseDatabase mDatabase;
    private DatabaseReference mRef;
    private FirebaseAuth mAuth;
    private String userCin;


    public MyService() {
    }

    @Override
    public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
        throw new UnsupportedOperationException("Not yet implemented");
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.e("TAG", "onStartCommand: ");
        initFirebase();

        mRef.child("user").child(mAuth.getCurrentUser().getUid()).addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                userCin = dataSnapshot.child("xxx").getValue().toString();
                mRef.child("parent").child("child").addChildEventListener(new ChildEventListener() {
                    @Override
                    public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                        showNotification();
                    }

                    @Override
                    public void onChildChanged(DataSnapshot dataSnapshot, String s) {

                    }

                    @Override
                    public void onChildRemoved(DataSnapshot dataSnapshot) {

                    }

                    @Override
                    public void onChildMoved(DataSnapshot dataSnapshot, String s) {

                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) {

                    }
                });
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
            }
        });

        return super.onStartCommand(intent, flags, startId);
    }

    private void showNotification() {
        Intent in = new Intent(this, MainActivity.class);
        PendingIntent pendingIntent = PendingIntent.getActivity(this, 1, in, PendingIntent.FLAG_UPDATE_CURRENT);

        NotificationCompat.Builder builder = new NotificationCompat.Builder(this)
                .setContentTitle("Title")
                .setContentText("Content")
                .setLargeIcon(BitmapFactory.decodeResource(getResources(), R.mipmap.logoapp))
                .setSmallIcon(R.mipmap.logoapp)
                .setSound(RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION))
                .setTicker("Ticker")
                .setContentIntent(pendingIntent)
                .setOnlyAlertOnce(true);
        NotificationManager manager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        manager.notify(1, builder.build());
    }

    private void initFirebase() {
        mDatabase = FirebaseDatabase.getInstance();
        mRef = mDatabase.getReference();
        mAuth = FirebaseAuth.getInstance();
    }


}

hello, this code is fired several times?