Primefaces 5.2, when menuitem should be evaluated/executed? - primefaces

Question

I am using JSF 2.2 with Primefaces 5.2 to implement my JSF project. I'm trying put a linkage with menuitem by using outcome to access my backing bean controller in my main.xhtml such as:
<h:form prependId="false" id="headerForm">
......
<p:menubar >
<p:submenu label="#{apps['application.staff.myinfo']}" icon="ui-icon-document">
<p:menuitem value="#{apps['application.staff.myinfo']}" immediate="true" outcome="#{staffControlBean.viewStaff()}" process="#form" update="#all" icon="fa fa-fw fa-leaf"
disabled="#{empty loginControlBean.staffSession.username}"/>
<p:separator />
<p:menuitem value="Quit" url="#" />
</p:submenu>
</p:menubar>
......
</h:form>
My backing bean as:
#Named(value = "staffControlBean")
#SessionScoped
public class StaffControlBean implements Serializable{
private static final String STAFF_VIEW="/staff/staffView.xhtml";
......
public String viewStaff(){
logger.info("View staff from menuitem clicked.");
if(loginControlBean.getStaffSession().isChangePassword()){
logger.info("password has to be changed before use this services.");
loginControlBean.fireChangePasswordDialog();
return "";
}else {
return STAFF_VIEW;
}
}
......
}
From above snippet you will see, I expect when I click the menuitem from the menubar, backing end controller method viewStaff will be invoked to return a change password dialog or return to my staff view.
But unfortunately, every time when I access (or refresh) main.xhtml without click any menuitem from menubar, the backing bean method was evaluated/executed twice. This is what I found from my server log:
......
####<Nov 10, 2016 2:02:29 PM AEDT> <Info> <com.longz.ozssc.web.staff.StaffControlBean> <macmini16g.tweedheadstorage.com.au> <OzsscWEBServer> <[ACTIVE] ExecuteThread: '2' for queue: 'weblogic.kernel.Default (self-tuning)'> <<anonymous>> <> <bc92beef-9f93-4923-a503-9a9cee37010d-00000071> <1478746949108> <[severity-value: 64] [rid: 0] [partition-id: 0] [partition-name: DOMAIN] > <BEA-000000> <View staff from menuitem clicked.>
####<Nov 10, 2016 2:02:29 PM AEDT> <Info> <com.longz.ozssc.web.staff.StaffControlBean> <macmini16g.tweedheadstorage.com.au> <OzsscWEBServer> <[ACTIVE] ExecuteThread: '2' for queue: 'weblogic.kernel.Default (self-tuning)'> <<anonymous>> <> <bc92beef-9f93-4923-a503-9a9cee37010d-00000071> <1478746949109> <[severity-value: 64] [rid: 0] [partition-id: 0] [partition-name: DOMAIN] > <BEA-000000> <View staff from menuitem clicked.>
......
My question is: When the linkage or buttons in the JSF will be evaluated or executed? Why it always been evaluated or executed when the page was loaded or refreshed. How I can avoid it happen?
Any advice are welcome and appreciated.