Multiple Call of Portlet RenderRequest

I have a Liferay (5.2.3) Portlet (2.0) with Vaadin (6.4.8), which usually works well.
Unfortunately, there is sometimes the following problem with the implementation of the handleRenderRequest function from PortletListener.
It seems that the user has called the function simultaneous (same millisecond!)! This leads to a ModificationException within the function. Could this be a problem wiht the PortletListeners?