I think the title is clear enough so I only add an example of typical situation.
First block of code:
<div id="mailpanel">
<h:panelGroup id="sendmailpane" styleClass="sendmailpane" layout="block"
rendered="#{userReports.reportRendered}">
<o:inputTextarea promptText="#{msg['mail.listrules']}" promptTextStyle="color: #333"
value="#{userReports.mailingList}" styleClass="maillist"/>
<br/>
<h:commandLink id="sendlink" value="#{msg['mail.sendLink']}"
action="#{userReports.sendMail}"/>
</h:panelGroup>
</div>
Second (copied) block of code:
<div id="mailpanel">
<h:panelGroup id="sendmailpane" styleClass="sendmailpane" layout="block"
rendered="#{projectReports.reportRendered}">
<o:inputTextarea promptText="#{msg['mail.listrules']}" promptTextStyle="color: #333"
value="#{projectReports.mailingList}" styleClass="maillist"/>
<br/>
<h:commandLink id="sendlink" value="#{msg['mail.sendLink']}"
action="#{projectReports.sendMail}"/>
</h:panelGroup>
</div>
As you can see both blocks of code are almost similar but each of them uses different backing bean (but even beans have a superclass and all used in this example methods are actually methods of that superclass).