### Question:
Cell implements NavComposite<Cell>, HasVisibility, HasFocusability, HasBounds { public ObservableList<Cell> children() { return new ExternalChildList(); } protected  Cell();   Cell(CellContainer cellContainer); static boolean isPopupProp(CellPropertySpec<?> prop); ObservableList<Cell> children(); @Override Cell nextSibling(); @Override Cell prevSibling(); @Override Cell firstChild(); @Override Cell lastChild(); Property<Boolean> hovered(); Property<Boolean> visible(); Property<Boolean> selected(); Property<Boolean> pairHighlighted(); Property<Boolean> focusHighlighted(); Property<Boolean> focusable(); Property<Color> background(); Property<Color> borderColor(); Property<Cell> topPopup(); Property<Cell> bottomPopup(); Property<Cell> frontPopup(); Property<Cell> leftPopup(); Property<Cell> rightPopup(); Property<Boolean> hasShadow(); Property<Boolean> hasPopupDecoration(); boolean isPopup(); void dispatch(EventT e, CellEventSpec<EventT> spec); void dispatch(EventT e, CellTraitEventSpec<EventT> spec); Registration addTrait(final CellTrait trait); @Override final Cell getParent(); final ReadableProperty<Cell> parent(); CellContainer getContainer(); ReadableProperty<CellContainer> cellContainer(); ReadableProperty<Boolean> focused(); void focus(); Vector origin(); Vector dimension(); Rectangle getBounds(); void scrollTo(); void scrollTo(Rectangle rect); Registration addListener(CellListener l); ValueT get(CellPropertySpec<ValueT> prop); Registration set(final CellPropertySpec<ValueT> prop, ValueT value); ValueT get(CellTraitPropertySpec<ValueT> prop); ValueT getRaw(CellTraitPropertySpec<ValueT> prop); Property<ValueT> getProp(final CellPropertySpec<ValueT> prop); boolean isAttached(); void removeFromParent(); Registration set(final CellTraitPropertySpec<ValueT> spec, final ValueT value); @Override String toString(); static final CellPropertySpec<Cell> TOP_POPUP; static final CellPropertySpec<Cell> BOTTOM_POPUP; static final CellPropertySpec<Cell> FRONT_POPUP; static final CellPropertySpec<Cell> LEFT_POPUP; static final CellPropertySpec<Cell> RIGHT_POPUP; static final List<CellPropertySpec<Cell>> POPUP_SPECS; static final CellPropertySpec<Boolean> POPUP; static final CellPropertySpec<Boolean> HAS_POPUP_DECORATION; static final CellPropertySpec<Boolean> HOVERED; static final CellPropertySpec<Boolean> FOCUSED; static final CellPropertySpec<Boolean> VISIBLE; static final CellPropertySpec<Boolean> SELECTED; static final CellPropertySpec<Boolean> FOCUS_HIGHLIGHTED; static final CellPropertySpec<Boolean> PAIR_HIGHLIGHTED; static final CellPropertySpec<Boolean> FOCUSABLE; static final CellPropertySpec<Color> BACKGROUND; static final CellPropertySpec<Color> BORDER_COLOR; static final CellPropertySpec<Boolean> HAS_SHADOW; static final CellPropertySpec<Boolean> RED_UNDERLINE; static final CellPropertySpec<Boolean> YELLOW_UNDERLINE; static final CellPropertySpec<Boolean> LINK; }### Answer:
@Test(expected = IllegalStateException.class) public void addingSameChildTwiceNotAllowed() { HorizontalCell parent = new HorizontalCell(); TextCell child = new TextCell(); parent.children().addAll(Arrays.asList(child, child)); }