diff --git a/Terminal.Gui/Core/MainLoop.cs b/Terminal.Gui/Core/MainLoop.cs
index 22bfa372a..c6bdbbd4f 100644
--- a/Terminal.Gui/Core/MainLoop.cs
+++ b/Terminal.Gui/Core/MainLoop.cs
@@ -31,7 +31,7 @@ namespace Terminal.Gui {
 		bool EventsPending (bool wait);
 
 		/// <summary>
-		/// The interation function.
+		/// The iteration function.
 		/// </summary>
 		void MainIteration ();
 	}
@@ -107,22 +107,30 @@ namespace Terminal.Gui {
 		///   Removes an idle handler added with <see cref="AddIdle(Func{bool})"/> from processing.
 		/// </summary>
 		/// <param name="token">A token returned by <see cref="AddIdle(Func{bool})"/></param>
-		public void RemoveIdle (Func<bool> token)
+		/// Returns <c>true</c>if the idle handler is successfully removed; otherwise, <c>false</c>.
+		///  This method also returns <c>false</c> if the idle handler is not found.
+		public bool RemoveIdle (Func<bool> token)
 		{
 			lock (token)
-				idleHandlers.Remove (token);
+				return idleHandlers.Remove (token);
 		}
 
 		void AddTimeout (TimeSpan time, Timeout timeout)
 		{
-			timeouts.Add ((DateTime.UtcNow + time).Ticks, timeout);
+			var k = (DateTime.UtcNow + time).Ticks;
+			lock (timeouts) {
+				while (timeouts.ContainsKey (k)) {
+					k = (DateTime.UtcNow + time).Ticks;
+				}
+			}
+			timeouts.Add (k, timeout);
 		}
 
 		/// <summary>
 		///   Adds a timeout to the mainloop.
 		/// </summary>
 		/// <remarks>
-		///   When time time specified passes, the callback will be invoked.
+		///   When time specified passes, the callback will be invoked.
 		///   If the callback returns true, the timeout will be reset, repeating
 		///   the invocation. If it returns false, the timeout will stop and be removed.
 		///
@@ -147,12 +155,15 @@ namespace Terminal.Gui {
 		/// <remarks>
 		///   The token parameter is the value returned by AddTimeout.
 		/// </remarks>
-		public void RemoveTimeout (object token)
+		/// Returns <c>true</c>if the timeout is successfully removed; otherwise, <c>false</c>.
+		/// This method also returns <c>false</c> if the timeout is not found.
+		public bool RemoveTimeout (object token)
 		{
 			var idx = timeouts.IndexOfValue (token as Timeout);
 			if (idx == -1)
-				return;
+				return false;
 			timeouts.RemoveAt (idx);
+			return true;
 		}
 
 		void RunTimers ()
@@ -160,8 +171,9 @@ namespace Terminal.Gui {
 			long now = DateTime.UtcNow.Ticks;
 			var copy = timeouts;
 			timeouts = new SortedList<long, Timeout> ();
-			foreach (var k in copy.Keys) {
-				var timeout = copy [k];
+			foreach (var t in copy) {
+				var k = t.Key;
+				var timeout = t.Value;
 				if (k < now) {
 					if (timeout.Callback (this))
 						AddTimeout (timeout.Span, timeout);
diff --git a/UnitTests/MainLoopTests.cs b/UnitTests/MainLoopTests.cs
index c4b69b14a..c85409021 100644
--- a/UnitTests/MainLoopTests.cs
+++ b/UnitTests/MainLoopTests.cs
@@ -5,6 +5,7 @@ using System.Globalization;
 using System.Linq;
 using System.Runtime.InteropServices.ComTypes;
 using System.Threading;
+using System.Threading.Tasks;
 using Terminal.Gui;
 using Xunit;
 using Xunit.Sdk;
@@ -33,28 +34,31 @@ namespace Terminal.Gui.Core {
 			ml.AddIdle (fnTrue);
 			ml.AddIdle (fnFalse);
 
-			ml.RemoveIdle (fnTrue);
+			Assert.True (ml.RemoveIdle (fnTrue));
 
-			// BUGBUG: This doens't throw or indicate an error. Ideally RemoveIdle would either 
-			// trhow an exception in this case, or return an error.
-			ml.RemoveIdle (fnTrue);
+			// BUGBUG: This doesn't throw or indicate an error. Ideally RemoveIdle would either 
+			// throw an exception in this case, or return an error.
+			// No. Only need to return a boolean.
+			Assert.False (ml.RemoveIdle (fnTrue));
 
-			ml.RemoveIdle (fnFalse);
+			Assert.True (ml.RemoveIdle (fnFalse));
 
 			// BUGBUG: This doesn't throw an exception or indicate an error. Ideally RemoveIdle would either 
-			// trhow an exception in this case, or return an error.
-			ml.RemoveIdle (fnFalse);
+			// throw an exception in this case, or return an error.
+			// No. Only need to return a boolean.
+			Assert.False (ml.RemoveIdle (fnFalse));
 
 			// Add again, but with dupe
 			ml.AddIdle (fnTrue);
 			ml.AddIdle (fnTrue);
 
-			ml.RemoveIdle (fnTrue);
-			ml.RemoveIdle (fnTrue);
+			Assert.True (ml.RemoveIdle (fnTrue));
+			Assert.True (ml.RemoveIdle (fnTrue));
 
 			// BUGBUG: This doesn't throw an exception or indicate an error. Ideally RemoveIdle would either 
-			// trhow an exception in this case, or return an error.
-			ml.RemoveIdle (fnTrue);
+			// throw an exception in this case, or return an error.
+			// No. Only need to return a boolean.
+			Assert.False (ml.RemoveIdle (fnTrue));
 		}
 
 		[Fact]
@@ -84,8 +88,7 @@ namespace Terminal.Gui.Core {
 				return true;
 			};
 
-			functionCalled = 0;
-			ml.RemoveIdle (fn);
+			Assert.False (ml.RemoveIdle (fn));
 			ml.MainIteration ();
 			Assert.Equal (0, functionCalled);
 		}
@@ -101,9 +104,8 @@ namespace Terminal.Gui.Core {
 				return true;
 			};
 
-			functionCalled = 0;
 			ml.AddIdle (fn);
-			ml.RemoveIdle (fn);
+			Assert.True (ml.RemoveIdle (fn));
 			ml.MainIteration ();
 			Assert.Equal (0, functionCalled);
 		}
@@ -119,21 +121,21 @@ namespace Terminal.Gui.Core {
 				return true;
 			};
 
-			functionCalled = 0;
 			ml.AddIdle (fn);
 			ml.AddIdle (fn);
 			ml.MainIteration ();
 			Assert.Equal (2, functionCalled);
 
 			functionCalled = 0;
-			ml.RemoveIdle (fn);
+			Assert.True (ml.RemoveIdle (fn));
 			ml.MainIteration ();
 			Assert.Equal (1, functionCalled);
 
 			functionCalled = 0;
-			ml.RemoveIdle (fn);
+			Assert.True (ml.RemoveIdle (fn));
 			ml.MainIteration ();
 			Assert.Equal (0, functionCalled);
+			Assert.False (ml.RemoveIdle (fn));
 		}
 
 		[Fact]
@@ -163,10 +165,11 @@ namespace Terminal.Gui.Core {
 			ml.AddIdle (fnStop);
 			ml.AddIdle (fn1);
 			ml.Run ();
-			ml.RemoveIdle (fnStop);
-			ml.RemoveIdle (fn1);
+			Assert.True (ml.RemoveIdle (fnStop));
+			Assert.False (ml.RemoveIdle (fn1));
 
 			Assert.Equal (10, functionCalled);
+			Assert.Equal (20, stopCount);
 		}
 
 		[Fact]
@@ -194,9 +197,9 @@ namespace Terminal.Gui.Core {
 			ml.AddIdle (fn1);
 			ml.AddIdle (fn1);
 			ml.Run ();
-			ml.RemoveIdle (fnStop);
-			ml.RemoveIdle (fn1);
-			ml.RemoveIdle (fn1);
+			Assert.True (ml.RemoveIdle (fnStop));
+			Assert.False (ml.RemoveIdle (fn1));
+			Assert.False (ml.RemoveIdle (fn1));
 
 			Assert.Equal (2, functionCalled);
 		}
@@ -217,7 +220,7 @@ namespace Terminal.Gui.Core {
 
 			ml.AddIdle (fn);
 			ml.Run ();
-			ml.RemoveIdle (fn);
+			Assert.True (ml.RemoveIdle (fn));
 
 			Assert.Equal (10, functionCalled);
 		}
@@ -237,10 +240,11 @@ namespace Terminal.Gui.Core {
 
 			var token = ml.AddTimeout (TimeSpan.FromMilliseconds (ms), callback);
 
-			ml.RemoveTimeout (token);
+			Assert.True (ml.RemoveTimeout (token));
 
 			// BUGBUG: This should probably fault?
-			ml.RemoveTimeout (token);
+			// Must return a boolean.
+			Assert.False (ml.RemoveTimeout (token));
 		}
 
 		[Fact]
@@ -258,11 +262,72 @@ namespace Terminal.Gui.Core {
 
 			var token = ml.AddTimeout (TimeSpan.FromMilliseconds (ms), callback);
 			ml.Run ();
-			ml.RemoveTimeout (token);
+			Assert.True (ml.RemoveTimeout (token));
 
 			Assert.Equal (1, callbackCount);
 		}
 
+		[Fact]
+		public async Task AddTimer_Duplicate_Keys_Not_Allowed ()
+		{
+			var ml = new MainLoop (new FakeMainLoop (() => FakeConsole.ReadKey (true)));
+			const int ms = 100;
+			object token1 = null, token2 = null;
+
+			var callbackCount = 0;
+			Func<MainLoop, bool> callback = (MainLoop loop) => {
+				callbackCount++;
+				if (callbackCount == 2) {
+					ml.Stop ();
+				}
+				return true;
+			};
+
+			var task1 = new Task (() => token1 = ml.AddTimeout (TimeSpan.FromMilliseconds (ms), callback));
+			var task2 = new Task (() => token2 = ml.AddTimeout (TimeSpan.FromMilliseconds (ms), callback));
+			Assert.Null (token1);
+			Assert.Null (token2);
+			task1.Start ();
+			task2.Start ();
+			ml.Run ();
+			Assert.NotNull (token1);
+			Assert.NotNull (token2);
+			await Task.WhenAll (task1, task2);
+			Assert.True (ml.RemoveTimeout (token1));
+			Assert.True (ml.RemoveTimeout (token2));
+
+			Assert.Equal (2, callbackCount);
+		}
+
+		[Fact]
+		public void AddTimer_In_Parallel_Wont_Throw ()
+		{
+			var ml = new MainLoop (new FakeMainLoop (() => FakeConsole.ReadKey (true)));
+			const int ms = 100;
+			object token1 = null, token2 = null;
+
+			var callbackCount = 0;
+			Func<MainLoop, bool> callback = (MainLoop loop) => {
+				callbackCount++;
+				if (callbackCount == 2) {
+					ml.Stop ();
+				}
+				return true;
+			};
+
+			Parallel.Invoke (
+				() => token1 = ml.AddTimeout (TimeSpan.FromMilliseconds (ms), callback),
+				() => token2 = ml.AddTimeout (TimeSpan.FromMilliseconds (ms), callback)
+			);
+			ml.Run ();
+			Assert.NotNull (token1);
+			Assert.NotNull (token2);
+			Assert.True (ml.RemoveTimeout (token1));
+			Assert.True (ml.RemoveTimeout (token2));
+
+			Assert.Equal (2, callbackCount);
+		}
+
 
 		class MillisecondTolerance : IEqualityComparer<TimeSpan> {
 			int _tolerance = 0;
@@ -293,7 +358,7 @@ namespace Terminal.Gui.Core {
 			// https://github.com/xunit/assert.xunit/pull/25
 			Assert.Equal<TimeSpan> (ms * callbackCount, watch.Elapsed, new MillisecondTolerance (100));
 
-			ml.RemoveTimeout (token);
+			Assert.True (ml.RemoveTimeout (token));
 			Assert.Equal (1, callbackCount);
 		}
 
@@ -321,7 +386,7 @@ namespace Terminal.Gui.Core {
 			// https://github.com/xunit/assert.xunit/pull/25
 			Assert.Equal<TimeSpan> (ms * callbackCount, watch.Elapsed, new MillisecondTolerance (100));
 
-			ml.RemoveTimeout (token);
+			Assert.True (ml.RemoveTimeout (token));
 			Assert.Equal (2, callbackCount);
 		}
 
@@ -349,7 +414,7 @@ namespace Terminal.Gui.Core {
 			};
 
 			var token = ml.AddTimeout (ms, callback);
-			ml.RemoveTimeout (token);
+			Assert.True (ml.RemoveTimeout (token));
 			ml.Run ();
 			Assert.Equal (0, callbackCount);
 		}
@@ -363,7 +428,7 @@ namespace Terminal.Gui.Core {
 			// Force stop if 10 iterations
 			var stopCount = 0;
 			Func<bool> fnStop = () => {
-				Thread.Sleep (10); // Sleep to enable timeer to fire
+				Thread.Sleep (10); // Sleep to enable timer to fire
 				stopCount++;
 				if (stopCount == 10) {
 					ml.Stop ();
@@ -382,7 +447,7 @@ namespace Terminal.Gui.Core {
 			ml.Run ();
 			Assert.Equal (1, callbackCount);
 			Assert.Equal (10, stopCount);
-			ml.RemoveTimeout (token);
+			Assert.False (ml.RemoveTimeout (token));
 		}
 
 		// Invoke Tests