diff --git a/OpenSim/Region/Framework/Scenes/ScenePresence.cs b/OpenSim/Region/Framework/Scenes/ScenePresence.cs
index 7243db119f..d7511d3034 100644
--- a/OpenSim/Region/Framework/Scenes/ScenePresence.cs
+++ b/OpenSim/Region/Framework/Scenes/ScenePresence.cs
@@ -108,6 +108,16 @@ namespace OpenSim.Region.Framework.Scenes
             }
         }
 
+        /// <summary>
+        /// This exists to prevent race conditions between two CompleteMovement threads if the simulator is slow and
+        /// the viewer fires these in quick succession.
+        /// </summary>
+        /// <remarks>
+        /// TODO: The child -> agent transition should be folded into LifecycleState and the CompleteMovement 
+        /// regulation done there.
+        /// </remarks>
+        private object m_completeMovementLock = new object();
+
 //        private static readonly byte[] DEFAULT_TEXTURE = AvatarAppearance.GetDefaultTexture().GetBytes();
         private static readonly Array DIR_CONTROL_FLAGS = Enum.GetValues(typeof(Dir_ControlFlags));
         private static readonly Vector3 HEAD_ADJUSTMENT = new Vector3(0f, 0f, 0.3f);
@@ -904,6 +914,7 @@ namespace OpenSim.Region.Framework.Scenes
         /// <summary>
         /// Turns a child agent into a root agent.
         /// </summary>
+        /// <remarks>
         /// Child agents are logged into neighbouring sims largely to observe changes.  Root agents exist when the
         /// avatar is actual in the sim.  They can perform all actions.
         /// This change is made whenever an avatar enters a region, whether by crossing over from a neighbouring sim,
@@ -911,8 +922,8 @@ namespace OpenSim.Region.Framework.Scenes
         ///
         /// This method is on the critical path for transferring an avatar from one region to another.  Delay here
         /// delays that crossing.
-        /// </summary>
-        private void MakeRootAgent(Vector3 pos, bool isFlying)
+        /// </remarks>
+        private bool MakeRootAgent(Vector3 pos, bool isFlying)
         {
 //            m_log.InfoFormat(
 //                "[SCENE]: Upgrading child to root agent for {0} in {1}",
@@ -920,6 +931,10 @@ namespace OpenSim.Region.Framework.Scenes
 
             //m_log.DebugFormat("[SCENE]: known regions in {0}: {1}", Scene.RegionInfo.RegionName, KnownChildRegionHandles.Count);
 
+            lock (m_completeMovementLock)
+                if (!IsChildAgent)
+                    return false;
+
             IsChildAgent = false;
 
             // Must reset this here so that a teleport to a region next to an existing region does not keep the flag
@@ -1069,6 +1084,7 @@ namespace OpenSim.Region.Framework.Scenes
 
             m_scene.EventManager.TriggerOnMakeRootAgent(this);
 
+            return true;
         }
 
         public int GetStateSource()
@@ -1442,7 +1458,14 @@ namespace OpenSim.Region.Framework.Scenes
             }
 
             bool flying = ((m_AgentControlFlags & AgentManager.ControlFlags.AGENT_CONTROL_FLY) != 0);
-            MakeRootAgent(AbsolutePosition, flying);
+            if (!MakeRootAgent(AbsolutePosition, flying))
+            {
+                m_log.DebugFormat(
+                    "[SCENE PRESENCE]: Aborting CompleteMovement call for {0} in {1} as they are already root", 
+                    Name, Scene.Name);
+
+                return;
+            }
 
             // Tell the client that we're totally ready
             ControllingClient.MoveAgentIntoRegion(m_scene.RegionInfo, AbsolutePosition, look);
diff --git a/OpenSim/Region/Framework/Scenes/Tests/ScenePresenceAgentTests.cs b/OpenSim/Region/Framework/Scenes/Tests/ScenePresenceAgentTests.cs
index d1aeaeef16..1ff1329d71 100644
--- a/OpenSim/Region/Framework/Scenes/Tests/ScenePresenceAgentTests.cs
+++ b/OpenSim/Region/Framework/Scenes/Tests/ScenePresenceAgentTests.cs
@@ -111,6 +111,45 @@ namespace OpenSim.Region.Framework.Scenes.Tests
             Assert.That(scene.GetScenePresences().Count, Is.EqualTo(1));
         }
 
+        /// <summary>
+        /// Test that duplicate complete movement calls are ignored.
+        /// </summary>
+        /// <remarks>
+        /// If duplicate calls are not ignored then there is a risk of race conditions or other unexpected effects.
+        /// </remarks>
+        [Test]
+        public void TestDupeCompleteMovementCalls()
+        {
+            TestHelpers.InMethod();
+//            TestHelpers.EnableLogging();
+
+            UUID spUuid = TestHelpers.ParseTail(0x1);
+
+            TestScene scene = new SceneHelpers().SetupScene();
+
+            int makeRootAgentEvents = 0;
+            scene.EventManager.OnMakeRootAgent += spi => makeRootAgentEvents++;
+
+            ScenePresence sp = SceneHelpers.AddScenePresence(scene, spUuid);
+
+            Assert.That(makeRootAgentEvents, Is.EqualTo(1));
+
+            // Normally these would be invoked by a CompleteMovement message coming in to the UDP stack.  But for
+            // convenience, here we will invoke it manually.
+            sp.CompleteMovement(sp.ControllingClient, true);
+
+            Assert.That(makeRootAgentEvents, Is.EqualTo(1));
+
+            // Check rest of exepcted parameters.
+            Assert.That(scene.AuthenticateHandler.GetAgentCircuitData(spUuid), Is.Not.Null);
+            Assert.That(scene.AuthenticateHandler.GetAgentCircuits().Count, Is.EqualTo(1));
+          
+            Assert.That(sp.IsChildAgent, Is.False);
+            Assert.That(sp.UUID, Is.EqualTo(spUuid));
+
+            Assert.That(scene.GetScenePresences().Count, Is.EqualTo(1));
+        }
+
         [Test]
         public void TestCreateDuplicateRootScenePresence()
         {
@@ -249,58 +288,5 @@ namespace OpenSim.Region.Framework.Scenes.Tests
 //            Assert.That(childPresence, Is.Not.Null);
 //            Assert.That(childPresence.IsChildAgent, Is.True);
         }
-
-//        /// <summary>
-//        /// Test adding a root agent to a scene.  Doesn't yet actually complete crossing the agent into the scene.
-//        /// </summary>
-//        [Test]
-//        public void T010_TestAddRootAgent()
-//        {
-//            TestHelpers.InMethod();
-//
-//            string firstName = "testfirstname";
-//
-//            AgentCircuitData agent = new AgentCircuitData();
-//            agent.AgentID = agent1;
-//            agent.firstname = firstName;
-//            agent.lastname = "testlastname";
-//            agent.SessionID = UUID.Random();
-//            agent.SecureSessionID = UUID.Random();
-//            agent.circuitcode = 123;
-//            agent.BaseFolder = UUID.Zero;
-//            agent.InventoryFolder = UUID.Zero;
-//            agent.startpos = Vector3.Zero;
-//            agent.CapsPath = GetRandomCapsObjectPath();
-//            agent.ChildrenCapSeeds = new Dictionary<ulong, string>();
-//            agent.child = true;
-//
-//            scene.PresenceService.LoginAgent(agent.AgentID.ToString(), agent.SessionID, agent.SecureSessionID);
-//
-//            string reason;
-//            scene.NewUserConnection(agent, (uint)TeleportFlags.ViaLogin, out reason);
-//            testclient = new TestClient(agent, scene);
-//            scene.AddNewAgent(testclient);
-//
-//            ScenePresence presence = scene.GetScenePresence(agent1);
-//
-//            Assert.That(presence, Is.Not.Null, "presence is null");
-//            Assert.That(presence.Firstname, Is.EqualTo(firstName), "First name not same");
-//            acd1 = agent;
-//        }
-//
-//        /// <summary>
-//        /// Test removing an uncrossed root agent from a scene.
-//        /// </summary>
-//        [Test]
-//        public void T011_TestRemoveRootAgent()
-//        {
-//            TestHelpers.InMethod();
-//
-//            scene.RemoveClient(agent1);
-//
-//            ScenePresence presence = scene.GetScenePresence(agent1);
-//
-//            Assert.That(presence, Is.Null, "presence is not null");
-//        }
     }
 }
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