[1.18.1] Storing items in ItemStack

[Squander]

When you right-click, the container and nests will open normally. But when he wants to put items in his backpack, the items start to duplicate. What did I do wrong?

public class BackpackItem extends Item implements MenuProvider, ICapabilityProvider {
    private final ItemStackHandler itemStackHandler = new ItemStackHandler(15);
    private final LazyOptional<IItemHandler> lazyOptional = LazyOptional.of(() -> itemStackHandler);

    public BackpackItem(Properties pProperties) {
        super(pProperties);
    }

    @Override
    public InteractionResultHolder<ItemStack> use(Level pLevel, Player pPlayer, InteractionHand hand) {
        if(!pLevel.isClientSide()){
            if(hand == InteractionHand.MAIN_HAND){
                NetworkHooks.openGui(((ServerPlayer) pPlayer), this);
            }
        }
        return InteractionResultHolder.success(pPlayer.getItemInHand(hand));
    }

    @Override
    public Component getDisplayName() {
        return new TextComponent("Backpack");
    }

    @Nullable
    @Override
    public AbstractContainerMenu createMenu(int pContainerId, Inventory pInventory, Player pPlayer) {
        return new BackpackMenu(pContainerId, pInventory, pPlayer.getMainHandItem());
    }

    @NotNull
    @Override
    public <T> LazyOptional<T> getCapability(@NotNull Capability<T> cap, @Nullable Direction side) {
        if(cap == CapabilityItemHandler.ITEM_HANDLER_CAPABILITY){
            return lazyOptional.cast();
        }
        return null;
    }

public class BackpackMenu extends AbstractContainerMenu {
    private final ItemStack backpack;
    private final Level level;

    public BackpackMenu(int id, Inventory inventory, FriendlyByteBuf buf){
        this(id, inventory, inventory.player.getMainHandItem());
    }

    public BackpackMenu(int pContainerId, Inventory inventory, ItemStack item) {
        super(ModMenuType.BACKPACK.get(), pContainerId);
        this.level = inventory.player.level;
        this.backpack = item;

        BackpackItem item1 = (BackpackItem) this.backpack.getItem();
        item1.getCapability(CapabilityItemHandler.ITEM_HANDLER_CAPABILITY).ifPresent(handler -> {
            for(int i = 0; i < 3; ++i) {
                for(int j = 0; j < 5; ++j) {
                    this.addSlot(new SlotItemHandler(handler, j + i * 3, 44 + j * 18, 16 + i * 18));
                }
            }
        });

        this.addPlayerHotbar(inventory);
        this.addPlayerInventory(inventory);
    }

    @Override
    public boolean stillValid(Player pPlayer) {
        return pPlayer.getMainHandItem().is(this.backpack.getItem());
    }

    private void addPlayerInventory(Inventory inv){
        for(int i = 0; i < 3; ++i) {
            for(int j = 0; j < 9; ++j) {
                this.addSlot(new Slot(inv, j + i * 9 + 9, 8 + j * 18, 84 + i * 18));
            }
        }
    }

    private void addPlayerHotbar(Inventory inv){
        for(int k = 0; k < 9; ++k) {
            this.addSlot(new Slot(inv, k, 8 + k * 18, 142));
        }
    }

    @Override
    public ItemStack quickMoveStack(Player pPlayer, int pIndex) {
        ItemStack toReturn = ItemStack.EMPTY;
        Slot slot = this.getSlot(pIndex);
        if(slot != null && slot.hasItem()){
            ItemStack stack = slot.getItem();
            toReturn = stack.copy();
            if(pIndex < 15){
                if(!this.moveItemStackTo(stack, 15, this.slots.size(), true)){
                    return ItemStack.EMPTY;
                }
            }else if(!this.moveItemStackTo(stack, 0, 15, false)){
                return ItemStack.EMPTY;
            }

            if(!stack.isEmpty()){
                slot.set(ItemStack.EMPTY);
            }else{
                slot.setChanged();
            }
        }
        return toReturn;
    }
}