Can't call externalContext.redirect with a cid inside

In my application, I need to redirect to another conversation.

So I do that :

@Inject
private HttpConversationContext conversationContext;

public void redirect() throws IOException {
	final ExternalContext externalContext = FacesContext.getCurrentInstance().getExternalContext();
	final Map<String, List<String>> parameters = new HashMap<String, List<String>>();
	final String conversationId = getConversationId();
	if (conversationId != null) {
		parameters.put(conversationContext.getParameterName(), Arrays.asList(conversationId));
	}
	//... put other parameters in the map
	externalContext.redirect(externalContext.encodeRedirectURL(externalContext.getRequestContextPath()+getViewId(), parameters));
}

(getConversationId() & getViewId() returns the cid number of the conversation I want to redirect to, and the faces viewId of my target page)

If I am in the conversation 2 & I want to redirect to conversation 1, here is the String passed to externalContext.redirect : "/mypage.xhtml?cid=1&otherparam=value"

The problem is that my browser is finally redirected to : "/mypage.xhtml?cid=2&cid=1&otherparam=value" !

I think that the problem comes from ConversationPropagationFilter.java:78 : in the FacesUrlTransformer, there is no check if in the http parameters, cid has already been specified or not.