Duplicate Id. JSF

Your problem is that these two components are part of the JSF Component tree for this page. And even if they cannot be displayed at the same time, they share the same ID, which is not allowed by JSF.

I see three solutions to solve your problem:

First solution: Define two differents ID

Second solution: You can, as explained by Wayne Young, use a NamingContainer, which will prefix their ID by the ID of the NamingContainer.

Third solution: Use only one <h:selectOneListbox/> and then make the difference in the Java code.

<h:selectOneListbox id="lang" size="5" disabled="#{!MbInstitution.managingNew}" value="#{MbInstitution.node.lang}" valueChangeListener="#{MbInstitution.changeLanguage}"> <a4j:support event="onchange" reRender="shortDesc, fullDesc" rendered="#{MbInstitution.view}"/> <f:selectItems value="#{MbInstitution.languages}"/> </h:selectOneListbox> 

Java code:

public List<SelectItem> getLanguage() { if (isView()) { return getLanguagesByInstitute(); } else { return getAvailableLanguages(); } } public void changeLanguage(ValueChangeEvent evt) { if (!isView()) { return; } ... }