Prevent race conditions between two threads that call LLClientView.Close() simultaneously (e.g. ack timeout and an attempt to reconnect)

OpenSim/Region/Framework/Scenes/Scene.cs

@@ -3517,8 +3517,8 @@ namespace OpenSim.Region.Framework.Scenes
                 // We have a zombie from a crashed session. 
                 // Or the same user is trying to be root twice here, won't work.
                 // Kill it.
-                m_log.DebugFormat(
-                    "[SCENE]: Zombie scene presence detected for {0} {1} in {2}",
+                m_log.WarnFormat(
+                    "[SCENE]: Existing root scene presence detected for {0} {1} in {2} when connecting.  Removing existing presence.",
                     sp.Name, sp.UUID, RegionInfo.RegionName);
 
                 sp.ControllingClient.Close();
@@ -4474,7 +4474,7 @@ namespace OpenSim.Region.Framework.Scenes
         /// </summary>
         /// <remarks>
         /// This method will return both root and child scene presences.
-        /// 
+        ///
         /// Consider using ForEachScenePresence() or ForEachRootScenePresence() if possible since these will not
         /// involving creating a new List object.
         /// </remarks>