A DESCRIPTION OF THE PROBLEM :
The issue shows up in a very simple test case containing an FXML file with a hierarchy of <ScrollPane><Group><Pagination> for which the pagination has an indeterminate number of pages. Although it is possible to select the final numbered page in the UI, a subsequent click on the ">" button causes an IllegalArgumentException to be thrown.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Build and run the test case.
2. Click on the final page number in the UI.
3. Click on the next button (">") in the UI.
4. The console displays an exception.


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The exception was not expected.
ACTUAL -
Exception in thread "JavaFX Application Thread" java.lang.IllegalArgumentExcepti
on: Children: duplicate children added: parent = HBox@a1af07e[styleClass=control
-box]
        at javafx.graphics/javafx.scene.Parent$3.onProposedChange(Parent.java:56
5)
        at javafx.base/com.sun.javafx.collections.VetoableListDecorator.add(Veto
ableListDecorator.java:206)
        at javafx.controls/javafx.scene.control.skin.PaginationSkin$NavigationCo
ntrol.initializePageIndicators(PaginationSkin.java:915)
        at javafx.controls/javafx.scene.control.skin.PaginationSkin$NavigationCo
ntrol.updatePageIndex(PaginationSkin.java:954)
        at javafx.controls/javafx.scene.control.skin.PaginationSkin$NavigationCo
ntrol.lambda$initializeNavigationHandlers$4(PaginationSkin.java:899)
        at javafx.base/com.sun.javafx.binding.ExpressionHelper$SingleChange.fire
ValueChangedEvent(ExpressionHelper.java:181)
        at javafx.base/com.sun.javafx.binding.ExpressionHelper.fireValueChangedE
vent(ExpressionHelper.java:80)
        at javafx.base/javafx.beans.property.IntegerPropertyBase.fireValueChange
dEvent(IntegerPropertyBase.java:107)
        at javafx.base/javafx.beans.property.IntegerPropertyBase.markInvalid(Int
egerPropertyBase.java:114)
        at javafx.base/javafx.beans.property.IntegerPropertyBase.set(IntegerProp
ertyBase.java:148)
        at javafx.controls/javafx.scene.control.Pagination.setCurrentPageIndex(P
agination.java:273)
        at javafx.controls/javafx.scene.control.skin.PaginationSkin.selectNext(P
aginationSkin.java:452)
        at javafx.controls/javafx.scene.control.skin.PaginationSkin.access$3100(
PaginationSkin.java:80)
        at javafx.controls/javafx.scene.control.skin.PaginationSkin$NavigationCo
ntrol.lambda$initializeNavigationHandlers$3(PaginationSkin.java:892)
        at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubbli
ngEvent(CompositeEventHandler.java:86)
        at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubbling
Event(EventHandlerManager.java:238)
        at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubbling
Event(EventHandlerManager.java:191)
        at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBub
blingEvent(CompositeEventDispatcher.java:59)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:58)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:56)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:56)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:56)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:56)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:56)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:56)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.ja
va:74)
        at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:4
9)
        at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
        at javafx.graphics/javafx.scene.Node.fireEvent(Node.java:8865)
        at javafx.controls/javafx.scene.control.Button.fire(Button.java:200)
        at javafx.controls/com.sun.javafx.scene.control.behavior.ButtonBehavior.
mouseReleased(ButtonBehavior.java:206)
        at javafx.controls/com.sun.javafx.scene.control.inputmap.InputMap.handle
(InputMap.java:274)
        at javafx.base/com.sun.javafx.event.CompositeEventHandler$NormalEventHan
dlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
        at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubbli
ngEvent(CompositeEventHandler.java:80)
        at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubbling
Event(EventHandlerManager.java:238)
        at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubbling
Event(EventHandlerManager.java:191)
        at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBub
blingEvent(CompositeEventDispatcher.java:59)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:58)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:56)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:56)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:56)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:56)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:56)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(B
asicEventDispatcher.java:56)
        at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent
(EventDispatchChainImpl.java:114)
        at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.ja
va:74)
        at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:5
4)
        at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
        at javafx.graphics/javafx.scene.Scene$MouseHandler.process(Scene.java:38
76)
        at javafx.graphics/javafx.scene.Scene$MouseHandler.access$1300(Scene.jav
a:3604)
        at javafx.graphics/javafx.scene.Scene.processMouseEvent(Scene.java:1874)

        at javafx.graphics/javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene
.java:2613)
        at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$Mouse
EventNotification.run(GlassViewEventHandler.java:397)
        at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$Mouse
EventNotification.run(GlassViewEventHandler.java:295)
        at java.base/java.security.AccessController.doPrivileged(Native Method)
        at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambd
a$handleMouseEvent$2(GlassViewEventHandler.java:434)
        at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRe
nderLock(QuantumToolkit.java:389)
        at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handl
eMouseEvent(GlassViewEventHandler.java:433)
        at javafx.graphics/com.sun.glass.ui.View.handleMouseEvent(View.java:556)

        at javafx.graphics/com.sun.glass.ui.View.notifyMouse(View.java:942)
        at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native M
ethod)
        at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(
WinApplication.java:175)
        at java.base/java.lang.Thread.run(Thread.java:844)


---------- BEGIN SOURCE ----------
>>> sample/Main.java

package sample;

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{
        Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));
        primaryStage.setTitle("Hello World");
        primaryStage.setScene(new Scene(root, 300, 275));
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}

>>> sample/sample.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import java.util.*?>
<?import javafx.scene.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>

<ScrollPane prefHeight="400" prefWidth="600" xmlns:fx="http://javafx.com/fxml/1">
    <Group>
        <Pagination prefHeight="200" prefWidth="200"/>
    </Group>
</ScrollPane>


---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
The exception can be avoided by replacing Group with the following class:

>>> sample/SampleGroup.java

package sample;

import javafx.beans.DefaultProperty;
import javafx.collections.ObservableList;
import javafx.scene.Node;
import javafx.scene.Parent;

@DefaultProperty("children")
public class SampleGroup extends Parent {
    @Override public ObservableList<Node> getChildren() {
        return super.getChildren();
    }
}


FREQUENCY : always