JSF 2.0 commandButton action called twice

I got a problem with JSF, hope someone can help me out. I have a Facelets xhtml page, with a SessionScoped managed bean. When a commandbutton is pressed on the page, a method is invoked in the bean, which dinamically creates another form on the page, with another commandbutton. That works very well so far, but when the newly created button is pressed on the page it somehow calls the action method twice. Here are the code samples from my backing bean:

The method which dynamically creates the new form, and works fine: (I used setActionExpression to bind an action to the button)

public void doCreateScreen() {
    FacesContext context = FacesContext.getCurrentInstance();
    Application application = context.getApplication();

    panelGrid.getChildren().clear();

    HtmlPanelGrid checkBoxGrid = (HtmlPanelGrid) application
            .createComponent(HtmlPanelGrid.COMPONENT_TYPE);
    checkBoxGrid.setColumns(columnNumber + 1);
    checkBoxGrid.getChildren().clear();

    for (int i = 1; i < rowNumber + 1; i++) {
        for (int j = 1; j < columnNumber + 1; j++) {
            HtmlSelectBooleanCheckbox tmpCheckBox = (HtmlSelectBooleanCheckbox) application
                    .createComponent(HtmlSelectBooleanCheckbox.COMPONENT_TYPE);
            String id = String.valueOf(i * columnNumber + j);
            tmpCheckBox.setId("box" + id);
            tmpCheckBox.setValue(true);
            checkBoxGrid.getChildren().add(tmpCheckBox);
        }
    }

    HtmlCommandButton createSeatsButton = (HtmlCommandButton) application
            .createComponent(HtmlCommandButton.COMPONENT_TYPE);
    createSeatsButton.setValue("Create Screen");

    MethodExpression createSeats = application.getExpressionFactory()
            .createMethodExpression(context.getELContext(),
                    "#{screencontroller.doCreateSeats}", null,
                    new Class<?>[0]);
    createSeatsButton.setActionExpression(createSeats);

    panelGrid.getChildren().add(checkBoxGrid);
    panelGrid.getChildren().add(createSeatsButton);

}

And this method which is called twice:

    public void doCreateSeats() {

        try {
            screen = screenOperator.createScreen(screen);

            for (int i = 1; i < rowNumber + 1; i++) {
                for (int j = 1; j < columnNumber + 1; j++) {
                    Seat seat = new Seat();
                    seat.setRow(i);
                    seat.setSeatNumber(j);
                    seat = screenOperator.createSeat(seat,
                            screen.getIdScreen());
                }
            }

            panelGrid.getChildren().clear();

        } catch (MovieTicketsException e) {
            // TODO
            e.printStackTrace();
        }

}

And the xhtml source:

        <div class="addform">
    <h1>Add new screen</h1>

    <h:form prependId="false">
        <h:panelGrid columns="2">
            <h:outputText value="Description:"></h:outputText>
            <h:inputText value="#{screencontroller.screen.description}"
                title="Description" id="Description" required="true"
                class="inputfield">
                <f:validateLength minimum="1" maximum="255"></f:validateLength>
            </h:inputText>


            <h:outputText value="Rows:"></h:outputText>
            <h:selectOneMenu value="#{screencontroller.rowNumber}" title="Rows"
                id="Rows" required="true" class="inputfield">
                <f:selectItems value="#{screencontroller.rowsSelectItems}">
                </f:selectItems>
            </h:selectOneMenu>

            <h:outputText value="Seats in a row:"></h:outputText>
            <h:selectOneMenu value="#{screencontroller.column开发者_如何学JAVANumber}"
                title="Columns" id="Columns" required="true" class="inputfield">
                <f:selectItems value="#{screencontroller.rowsSelectItems}">
                </f:selectItems>
            </h:selectOneMenu>

            <h:outputLabel></h:outputLabel>
            <h:commandButton action="#{screencontroller.doCreateScreen}"
                value="Submit"></h:commandButton>

        </h:panelGrid>
    </h:form> <h:message for="Description" class="errormsg"></h:message> <h:message
        for="Cinema" class="errormsg"></h:message></div>

    <div class="listdiv"><h:form prependId="false">
        <h:panelGrid binding="#{screencontroller.panelGrid}" columns="1">
        </h:panelGrid>
    </h:form></div>

    </div>

I've just started learning jsf, so maybe i'm doing somthing really wrong, but i would be very thankful if someone could explain this.

Thanks