flow: Confirmed duplicate message from the client if a Throwable is thrown

Example

@Route("")
@Theme(Lumo.class)
public class DemoView extends VerticalLayout {

    public DemoView() {
        getElement().addEventListener("click", e -> {
            throw new Error("Fail");
        });
}

Clicking on the button logs two exceptions

[WARNING] Error for /
java.lang.Error: Fail
	at DemoView.lambda$0(DemoView.java:14)
	at com.vaadin.flow.internal.nodefeature.ElementListenerMap.lambda$fireEvent$0(ElementListenerMap.java:171)
	at java.util.ArrayList.forEach(ArrayList.java:1249)
	at com.vaadin.flow.internal.nodefeature.ElementListenerMap.fireEvent(ElementListenerMap.java:171)
	at com.vaadin.flow.server.communication.rpc.EventRpcHandler.handleNode(EventRpcHandler.java:58)
	at com.vaadin.flow.server.communication.rpc.AbstractRpcInvocationHandler.handle(AbstractRpcInvocationHandler.java:60)
	at com.vaadin.flow.server.communication.ServerRpcHandler.handleInvocationData(ServerRpcHandler.java:377)
	at com.vaadin.flow.server.communication.ServerRpcHandler.lambda$handleInvocations$0(ServerRpcHandler.java:367)
	at java.util.ArrayList.forEach(ArrayList.java:1249)
	at com.vaadin.flow.server.communication.ServerRpcHandler.handleInvocations(ServerRpcHandler.java:367)
	at com.vaadin.flow.server.communication.ServerRpcHandler.handleRpc(ServerRpcHandler.java:309)
	at com.vaadin.flow.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:89)
	at com.vaadin.flow.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:40)
	at com.vaadin.flow.server.VaadinService.handleRequest(VaadinService.java:1541)
	at com.vaadin.flow.server.VaadinServlet.service(VaadinServlet.java:345)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)

which is expected but then the client side gets a 500 back. The client side then tries to re-send the message as it does not know if the message ever reached the server. This will cause another exception to be logged

[qtp1532675992-33] ERROR com.vaadin.flow.server.DefaultErrorHandler - 
java.lang.UnsupportedOperationException: Confirmed duplicate message from the client. Expected sync id: 1, got 0. Message start: {"csrfToken":"ab0f5d03-ba71-408a-8e60-9bfc09b8df65","rpc":[{"type":"event","node":3,"event":"click"}],"syncId":0,"clientId":0}
	at com.vaadin.flow.server.communication.ServerRpcHandler.handleRpc(ServerRpcHandler.java:302)
	at com.vaadin.flow.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:89)
	at com.vaadin.flow.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:40)
	at com.vaadin.flow.server.VaadinService.handleRequest(VaadinService.java:1541)
	at com.vaadin.flow.server.VaadinServlet.service(VaadinServlet.java:345)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
	at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:835)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1685)
	at org.eclipse.jetty.websocket.server.WebSocketUpgradeFilter.doFilter(WebSocketUpgradeFilter.java:225)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1668)

In FW8, this case is handled as

            ui.getSession().getCommunicationManager().repaintAll(ui);

            if (rpcRequest.getClientToServerId() < expectedId) {
                // Just a duplicate message due to a bad connection or similar
                // It has already been handled by the server so it is safe to
                // ignore
                getLogger()
                        .fine("Ignoring old message from the client. Expected: "
                                + expectedId + ", got: "
                                + rpcRequest.getClientToServerId());

which seems more appropriate.

In real world, this happened during development when part of the code was not redeployed and caused a java.lang.NoSuchMethodError to be thrown