Исключение в потоке «JavaFX Application Thread» java.lang.IllegalArgumentException:

Вот ошибка

Exception in thread "JavaFX Application Thread" java.lang.IllegalArgumentException: Children: duplicate children added: parent = Group@5ea0c6fc
at javafx.graphics/javafx.scene.Parent$3.onProposedChange(Unknown Source)
at javafx.base/com.sun.javafx.collections.VetoableListDecorator.add(Unknown Source)
at project1.Crab.setCrab(Crab.java:45)
at project1.SeaAnimals.addCrab(SeaAnimals.java:65)
at project1.ProjectOne.mouseClick(ProjectOne.java:133)
at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(Unknown Source)
at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(Unknown Source)
at javafx.base/javafx.event.Event.fireEvent(Unknown Source)
at javafx.graphics/javafx.scene.Scene$ClickGenerator.postProcess(Unknown Source)
at javafx.graphics/javafx.scene.Scene$ClickGenerator.access$8300(Unknown Source)
at javafx.graphics/javafx.scene.Scene$MouseHandler.process(Unknown Source)
at javafx.graphics/javafx.scene.Scene$MouseHandler.access$1300(Unknown Source)
at javafx.graphics/javafx.scene.Scene.processMouseEvent(Unknown Source)
at javafx.graphics/javafx.scene.Scene$ScenePeerListener.mouseEvent(Unknown Source)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(Unknown Source)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(Unknown Source)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$2(Unknown Source)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(Unknown Source)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(Unknown Source)
at javafx.graphics/com.sun.glass.ui.View.handleMouseEvent(Unknown Source)
at javafx.graphics/com.sun.glass.ui.View.notifyMouse(Unknown Source)
at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(Unknown Source)
at java.base/java.lang.Thread.run(Unknown Source)

Вот строки кода, в которых есть проблема

public class Crab {

    private final Image c = new Image("File:crab.gif");
    private ArrayList<ImageView> crabList = new ArrayList<ImageView>();
    private int _counter;
    private Group _crab = new Group();
    /** Constructor <br>
     * Initializes counter to -1
     */
    public Crab() {
        _counter = -1;
    }
    /** Setter <br>
     * Adds a crab to the array list
     */
    public void addCrab() {
        if (_counter < 5) {
            crabList.add(new ImageView(c));
            _counter++;
        }
    }
    /** Setter <br> 
     * Deletes a crab from the array list
     */
    public void deleteCrab() {
        if (_counter > 0) {
            crabList.remove(_counter);
            _counter--;
        }
    }
    /** Getter <br>
     * 
     * @return returns crab group
     */
    public Group setCrab() {
        if (_counter >= 0) { //adds the first crab
            crabList.get(0).setTranslateX(80);
            crabList.get(0).setTranslateY(500);
            crabList.get(0).setFitHeight(100);
            crabList.get(0).setFitWidth(100);
            _crab.getChildren().add(crabList.get(0));
            if (_counter >= 1) { //adds the second crab
                crabList.get(1).setTranslateX(230);
                crabList.get(1).setTranslateY(500);
                crabList.get(1).setFitHeight(100);
                crabList.get(1).setFitWidth(100);
                _crab.getChildren().add(crabList.get(1));       
                if (_counter >= 2) { // adds the third crab
                    crabList.get(2).setTranslateX(380);
                    crabList.get(2).setTranslateY(500);
                    crabList.get(2).setFitHeight(100);
                    crabList.get(2).setFitWidth(100);
                    _crab.getChildren().add(crabList.get(2));
                    if (_counter >= 3) { //adds the fourth crab
                        crabList.get(3).setTranslateX(530);
                        crabList.get(3).setTranslateY(500);
                        crabList.get(3).setFitHeight(100);
                        crabList.get(3).setFitWidth(100);
                        _crab.getChildren().add(crabList.get(3));   
                        if (_counter >= 4) { //adds the fifth crab
                            crabList.get(4).setTranslateX(680);
                            crabList.get(4).setTranslateY(500);
                            crabList.get(4).setFitHeight(100);
                            crabList.get(4).setFitWidth(100);
                            _crab.getChildren().add(crabList.get(4));           
                            if (_counter == 5) { //adds the sixth crab
                                crabList.get(5).setTranslateX(830);
                                crabList.get(5).setTranslateY(500);
                                crabList.get(5).setFitHeight(100);
                                crabList.get(5).setFitWidth(100);
                                _crab.getChildren().add(crabList.get(5));
                            }
                        }
                    }
                }
            }
        }
        return _crab;
    }
}

    public Group addCrab() {
        _crab.addCrab();
        _gCrab = _crab.setCrab();
        return _gCrab;
    }
    public Group deleteCrab() {
        _crab.deleteCrab();
        _gCrab = _crab.setCrab();
        return _gCrab;
    }
}

public void mouseClick(MouseEvent mouse) {
    if (mouse.getClickCount() == 1) {
        root.getChildren().remove(gCrab);
        gCrab.getChildren().clear();
        gCrab.getChildren().add(animals.addCrab());
        root.getChildren().add(gCrab);
    }
    else if (mouse.getClickCount() == 2) {
        root.getChildren().remove(gCrab);
        gCrab.getChildren().clear();
        gCrab.getChildren().add(animals.deleteCrab());
        root.getChildren().add(gCrab);
    }
}

Я не знаю, как решить проблему, и не знаю, в чем проблема. Любая помощь приветствуется.