Prevent users from connecting many times under some circumstances.

src/main/java/net/md_5/bungee/InitialHandler.java

@@ -59,6 +59,12 @@ public class InitialHandler implements Runnable
                         throw new KickException("Not authenticated with minecraft.net");
                     }
 
+                    // Check for multiple connections
+                    if (BungeeCord.instance.connections.containsKey(handshake.username))
+                    {
+                        throw new KickException("You are already connected to the server");
+                    }
+
                     // fire post auth event
                     BungeeCord.instance.pluginManager.onLogin(event);
                     if (event.isCancelled())