Java 在recyclerview中从firebase获取数据时，一次又一次地获取重复数据

我正在从firebase获取数据。首先，我只获取10个项目，然后当用户滚动recyclerview时，接下来的10个项目正在获取。 现在我的问题是，当我滚动recyclerview时，我得到了重复的数据。我不知道为什么会这样。为什么要再次获取相同的数据。 请看我的代码，告诉我我的错误

当我第一次仅获取10个项目时：

content = getIntent().getStringExtra("jokecontent");
       funnyJokesQuery = FirebaseDatabase.getInstance().getReference("ContentJokes").orderByChild("subcategory")
               .equalTo(content).limitToFirst(10);
       funnyJokesQuery.addChildEventListener(new ChildEventListener() {
           @Override
           public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
               String contentdisplay = dataSnapshot.child("content").getValue(String.class);
               ContentModel contentModel = new ContentModel(contentdisplay);
               contentname.add(0,contentModel);
               arrayAdapter.notifyDataSetChanged();
               progressBar.setVisibility(View.GONE);
               arrayAdapter.notifyItemInserted(0);
           }
           @Override
           public void onChildChanged(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

           }

           @Override
           public void onChildRemoved(@NonNull DataSnapshot dataSnapshot) {

           }

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

           }

           @Override
           public void onCancelled(@NonNull DatabaseError databaseError) {

           }


       });

recyclerview的onScrollListener用于获取下一个项目的代码：

 recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
        @Override
        public void onScrollStateChanged(@NonNull RecyclerView recyclerView, int newState) {
            super.onScrollStateChanged(recyclerView, newState);

            if(newState == AbsListView.OnScrollListener.SCROLL_STATE_TOUCH_SCROLL)
            {
                isScrolling = true;
            }
        }

        @Override
        public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
            super.onScrolled(recyclerView, dx, dy);
            currentitems = manager.getChildCount();
            totalitems = manager.getItemCount();
            scrollOutItems = manager.findFirstVisibleItemPosition();
            lastItemPosition = manager.findLastVisibleItemPosition();
            if(isScrolling &&(currentitems +scrollOutItems == totalitems)){
                    funnyJokesQuery = FirebaseDatabase.getInstance().getReference("ContentJokes")
                            .orderByChild("subcategory")
                            .startAt(lastItemPosition)
                            .limitToFirst(11);
                    progressBar.setVisibility(View.VISIBLE);
                    funnyJokesQuery.addChildEventListener(new ChildEventListener() {
                        @Override
                        public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
                            String contentdisplay = dataSnapshot.child("content").getValue(String.class);
                            ContentModel contentModel = new ContentModel(contentdisplay);
                            contentname.add(lastItemPosition + 1, contentModel);
                            arrayAdapter.notifyDataSetChanged();
                            progressBar.setVisibility(View.GONE);

                        }

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


                        }

                        @Override
                        public void onChildRemoved(@NonNull DataSnapshot dataSnapshot) {

                        }

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

                        }

                        @Override
                        public void onCancelled(@NonNull DatabaseError databaseError) {

                        }


                    });
                }

            }
        }
    });