Problems with rich:dropDownMenu

[liar]

hi,
I am using rich:dropDownMenu. I find that ,all the method in the page that the rich:dropDownMenu nagvigated to are called two time. SO ,if you click the menu Item the JSF commponet like:

<h:panelGrid columns="1">
 <h:selectOneMenu id="projectlist" value="#{search.project}">
 <f:selectItems value="#{search.projectList}" />
 </h:selectOneMenu>
 </h:panelGrid>

may run two times.
Thank you very much in advance!