Property:

org.zkoss.zk.ui.EventListener.duplicateIgnored

Default: true: allows one instance of event listener to be added multiple times to the listener list of a component or page.

If set to false, components and pages will check their internal listener list for the same instance of listener, and will return false instead of adding it again if found.

<library-property>
    <name>org.zkoss.zk.ui.EventListener.duplicateIgnored</name>
    <value>true</value>
</library-property>