add only one value to a picklist target (PrimeFaces) - jsf-2

Question

I have a search function which searches the system. After performing search I am updating the search result in to pick List. Each Time I search I will update pickList source. Now My problem is I want to validate in such a way that if more than 1 value is added to the pick list target It should give me error message and also check about the duplicate entries in picklist target.
Below is the code of picklist in xhmtl
<p:pickList id="primaryContactBean" value="#{addSystemManagedBean.primaryContactList}" var="contact" itemLabel="#{contact.firstName}" itemValue="#{contact}" converter="contactConverter" >
<p:ajax event="transfer" listener="#{addSystemManagedBean.onTransfer}" process="#this" update="sysMsg3" />
<p:column>
#{contact.firstName} #{contact.lastName}
</p:column>
</p:pickList>
and onTransfer the following method gets called
public int primaryOwnerCount = 0;
public void onTransfer(TransferEvent event) {
PickList picklist = (PickList) event.getComponent();
for(Object item : event.getItems()) {
ContactBean cb = (ContactBean)item;
if(picklist.getId().equals("primaryContactBean")){
if(event.isAdd())
primaryOwnerCount++;
if(primaryOwnerCount>1){
FacesContext.getCurrentInstance().addMessage(null, new FacesMessage(FacesMessage.SEVERITY_ERROR,"Only one primary owner can exist!!","If you want to add owner, please remove from the list and then proceed.."));
for (Iterator iterator = primaryContactList.getTarget().iterator(); iterator
.hasNext();) {
ContactBean type = (ContactBean) iterator.next();
System.out.println("target:"+type.getFirstName());
}
}
else
primaryContactTarget.add(cb);
}