diff --git a/src/InProcessTestHost/InProcessTestHost.csproj b/src/InProcessTestHost/InProcessTestHost.csproj
index 239338158..9cdc750c0 100644
--- a/src/InProcessTestHost/InProcessTestHost.csproj
+++ b/src/InProcessTestHost/InProcessTestHost.csproj
@@ -5,7 +5,7 @@
     <RootNamespace>Microsoft.DurableTask.Testing</RootNamespace>
     <AssemblyName>Microsoft.DurableTask.InProcessTestHost</AssemblyName>
     <PackageId>Microsoft.DurableTask.InProcessTestHost</PackageId>
-    <Version>0.2.0-preview.1</Version>
+    <Version>0.2.1-preview.1</Version>
     
     <!-- Suppress CA1848: Use LoggerMessage delegates for high-performance logging scenarios -->
     <NoWarn>$(NoWarn);CA1848</NoWarn>
diff --git a/src/InProcessTestHost/Sidecar/InMemoryOrchestrationService.cs b/src/InProcessTestHost/Sidecar/InMemoryOrchestrationService.cs
index 40691bd91..17d69c5e3 100644
--- a/src/InProcessTestHost/Sidecar/InMemoryOrchestrationService.cs
+++ b/src/InProcessTestHost/Sidecar/InMemoryOrchestrationService.cs
@@ -879,7 +879,7 @@ public PurgeResult PurgeInstanceState(PurgeInstanceFilter purgeInstanceFilter)
         class ReadyToRunQueue
         {
             readonly Channel<SerializedInstanceState> readyToRunQueue = Channel.CreateUnbounded<SerializedInstanceState>();
-            readonly Dictionary<string, object> readyInstances = new(StringComparer.OrdinalIgnoreCase);
+            readonly ConcurrentDictionary<string, SerializedInstanceState> readyInstances = new(StringComparer.OrdinalIgnoreCase);
 
             public void Reset()
             {
@@ -893,7 +893,7 @@ public async ValueTask<SerializedInstanceState> TakeNextAsync(CancellationToken
                     SerializedInstanceState state = await this.readyToRunQueue.Reader.ReadAsync(ct);
                     lock (state)
                     {
-                        if (this.readyInstances.Remove(state.InstanceId))
+                        if (this.readyInstances.TryRemove(state.InstanceId, out _))
                         {
                             if (state.IsLoaded)
                             {
@@ -909,12 +909,9 @@ public async ValueTask<SerializedInstanceState> TakeNextAsync(CancellationToken
 
             public void Schedule(SerializedInstanceState state)
             {
-                // TODO: There is a race condition here. If another thread is calling TakeNextAsync
-                //       and removed the queue item before updating the dictionary, then we'll fail
-                //       to update the readyToRunQueue and the orchestration will get stuck.
                 if (this.readyInstances.TryAdd(state.InstanceId, state))
                 {
-                    if (!this.readyToRunQueue.Writer.TryWrite(state)) 
+                    if (!this.readyToRunQueue.Writer.TryWrite(state))
                     {
                         throw new InvalidOperationException($"unable to write to queue for {state.InstanceId}");
                     }
diff --git a/test/InProcessTestHost.Tests/ConcurrentTimerTests.cs b/test/InProcessTestHost.Tests/ConcurrentTimerTests.cs
new file mode 100644
index 000000000..75a7ef3e3
--- /dev/null
+++ b/test/InProcessTestHost.Tests/ConcurrentTimerTests.cs
@@ -0,0 +1,104 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using Microsoft.DurableTask;
+using Microsoft.DurableTask.Client;
+using Microsoft.DurableTask.Testing;
+using Microsoft.DurableTask.Worker;
+using Xunit;
+
+namespace InProcessTestHost.Tests;
+
+/// <summary>
+/// Tests to verify that multiple orchestrations with identical timer FireAt timestamps
+/// all complete correctly without any being dropped.
+/// </summary>
+public class ConcurrentTimerTests
+{
+    [Fact]
+    // Test that multiple orchestrations with the same timer that fire at the same time
+    // can all complete correctly.
+    public async Task MultipleOrchestrations_WithSameTimerFireAt_AllComplete()
+    {
+        const int orchestrationCount = 10;
+        const string orchestratorName = "TimerOrchestrator";
+
+        await using DurableTaskTestHost host = await DurableTaskTestHost.StartAsync(tasks =>
+        {
+            tasks.AddOrchestratorFunc<DateTime, string>(orchestratorName, async (ctx, fireAt) =>
+            {
+                await ctx.CreateTimer(fireAt, CancellationToken.None);
+                return $"done:{ctx.InstanceId}";
+            });
+        });
+
+        DateTime sharedFireAt = DateTime.UtcNow.AddSeconds(5);
+
+        string[] instanceIds = new string[orchestrationCount];
+        for (int i = 0; i < orchestrationCount; i++)
+        {
+            instanceIds[i] = await host.Client.ScheduleNewOrchestrationInstanceAsync(
+                orchestratorName, sharedFireAt);
+        }
+
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(30));
+
+        Task<OrchestrationMetadata>[] waitTasks = instanceIds
+            .Select(id => host.Client.WaitForInstanceCompletionAsync(
+                id, getInputsAndOutputs: true, cts.Token))
+            .ToArray();
+
+        OrchestrationMetadata[] results = await Task.WhenAll(waitTasks);
+
+        for (int i = 0; i < orchestrationCount; i++)
+        {
+            Assert.NotNull(results[i]);
+            Assert.Equal(OrchestrationRuntimeStatus.Completed, results[i].RuntimeStatus);
+            string output = results[i].ReadOutputAs<string>()!;
+            Assert.Equal($"done:{instanceIds[i]}", output);
+        }
+    }
+
+    [Fact]
+    // Test that fan-out sub-orchestrations with timers that all fire at the same time
+    // can all complete correctly.
+    public async Task SubOrchestrations_WithIdenticalTimers_AllComplete()
+    {
+        const int subOrchestrationCount = 10;
+        const string parentName = "ParentOrchestrator";
+        const string childName = "ChildTimerOrchestrator";
+
+        await using DurableTaskTestHost host = await DurableTaskTestHost.StartAsync(tasks =>
+        {
+            tasks.AddOrchestratorFunc<int>(parentName, async ctx =>
+            {
+                DateTime sharedFireAt = ctx.CurrentUtcDateTime.AddSeconds(2);
+
+                // A parent orchestration will schedule 10 sub-orchestrations which has a timer
+                // fires at the same time.
+                Task<string>[] childTasks = Enumerable.Range(0, subOrchestrationCount)
+                    .Select(i => ctx.CallSubOrchestratorAsync<string>(childName, sharedFireAt))
+                    .ToArray();
+
+                string[] results = await Task.WhenAll(childTasks);
+                return results.Length;
+            });
+
+            tasks.AddOrchestratorFunc<DateTime, string>(childName, async (ctx, fireAt) =>
+            {
+                await ctx.CreateTimer(fireAt, CancellationToken.None);
+                return $"child-done:{ctx.InstanceId}";
+            });
+        });
+
+        string instanceId = await host.Client.ScheduleNewOrchestrationInstanceAsync(parentName);
+
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(60));
+        OrchestrationMetadata metadata = await host.Client.WaitForInstanceCompletionAsync(
+            instanceId, getInputsAndOutputs: true, cts.Token);
+
+        Assert.NotNull(metadata);
+        Assert.Equal(OrchestrationRuntimeStatus.Completed, metadata.RuntimeStatus);
+        Assert.Equal(subOrchestrationCount, metadata.ReadOutputAs<int>());
+    }
+}