title: Vaadin "A connector with id xy is already registered"
body: <p>Somewhere in my Vaadin application, I'm getting this exception as soon as I connect using a second browser</p>
  
  <p><code>Caused by: java.lang.RuntimeException: A connector with id 22 is already registered!
      at com.vaadin.ui.ConnectorTracker.registerConnector(ConnectorTracker.java:133)</code></p>
  
  <p>It happens always in the same place but I don't know why exactly as the reason for this must be somewhere else.</p>
  
  <p>I think I might be stealing UI components from the other session - which is not my intention.
  Currently, I don't see any <code>static</code> instances of UI components I might be using in multiple sessions.</p>
  
  <p>How can I debug this? It's become quite a large project.
  Any hints to look for?</p>