diff --git a/dotnet/src/Microsoft.Agents.AI.DurableTask/CHANGELOG.md b/dotnet/src/Microsoft.Agents.AI.DurableTask/CHANGELOG.md
index 463b792..ecc8176 100644
--- a/dotnet/src/Microsoft.Agents.AI.DurableTask/CHANGELOG.md
+++ b/dotnet/src/Microsoft.Agents.AI.DurableTask/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 
+- Fixed `AddWorkflow` silently overwriting an existing workflow registered under the same name, which left the workflow and executor registries inconsistent. Registering a different workflow under a name that is already taken now throws, while re-registering the same workflow instance remains a no-op ([#66](https://github.com/microsoft/agent-framework-durable-extension/pull/66))
 - Fixed a `JsonTypeInfo metadata ... was not provided` failure when persisting agent state for function calls or results that carry values the state serializer has no metadata for, such as the `AIContent` results returned by MCP tools ([#57](https://github.com/microsoft/agent-framework-durable-extension/pull/57))
 - [BREAKING] Added `IWorkflowClient` overloads that start a registered workflow by name, and made workflow result deserialization case-insensitive so results can be read back when hosted in Azure Functions. External implementations of `IWorkflowClient` must implement the new members, and an untyped `null` first argument is now ambiguous between the `Workflow` and workflow-name overloads ([#48](https://github.com/microsoft/agent-framework-durable-extension/pull/48))
 - [BREAKING] Removed the `AddAIAgents` and `AddWorkflows` bulk registration APIs and changed `AddWorkflow` to return `DurableWorkflowOptions` so multiple workflows can be registered fluently ([#39](https://github.com/microsoft/agent-framework-durable-extension/pull/39))
diff --git a/dotnet/src/Microsoft.Agents.AI.DurableTask/Workflows/DurableWorkflowOptions.cs b/dotnet/src/Microsoft.Agents.AI.DurableTask/Workflows/DurableWorkflowOptions.cs
index 8222cc4..7d9b916 100644
--- a/dotnet/src/Microsoft.Agents.AI.DurableTask/Workflows/DurableWorkflowOptions.cs
+++ b/dotnet/src/Microsoft.Agents.AI.DurableTask/Workflows/DurableWorkflowOptions.cs
@@ -43,12 +43,21 @@ internal DurableWorkflowOptions(DurableOptions? parentOptions = null)
     /// <param name="workflow">The workflow instance to add. Cannot be null.</param>
     /// <returns>The options instance, so that multiple calls can be chained.</returns>
     /// <remarks>
+    /// <para>
     /// When a workflow is added, all executors are registered in the executor registry.
     /// Any AI agent executors will also be automatically registered with the
     /// <see cref="DurableAgentsOptions"/> if available.
+    /// </para>
+    /// <para>
+    /// Workflow names must be unique because they identify the orchestration that runs the workflow.
+    /// Adding the same workflow instance more than once is a no-op, so a sub-workflow that is also
+    /// registered explicitly is not reported as a conflict when it is discovered during registration.
+    /// </para>
     /// </remarks>
     /// <exception cref="ArgumentNullException">Thrown when <paramref name="workflow"/> is null.</exception>
-    /// <exception cref="ArgumentException">Thrown when the workflow does not have a valid name.</exception>
+    /// <exception cref="ArgumentException">
+    /// Thrown when the workflow does not have a valid name, or when a different workflow with the same name has already been registered.
+    /// </exception>
     public DurableWorkflowOptions AddWorkflow(Workflow workflow)
     {
         ArgumentNullException.ThrowIfNull(workflow);
@@ -58,6 +67,17 @@ public DurableWorkflowOptions AddWorkflow(Workflow workflow)
             throw new ArgumentException("Workflow must have a valid Name property.", nameof(workflow));
         }
 
+        if (this._workflows.TryGetValue(workflow.Name, out Workflow? existingWorkflow))
+        {
+            if (!ReferenceEquals(existingWorkflow, workflow))
+            {
+                throw new ArgumentException($"A workflow with name '{workflow.Name}' has already been registered.", nameof(workflow));
+            }
+
+            // The same instance was already registered, so its executors are registered too.
+            return this;
+        }
+
         this._workflows[workflow.Name] = workflow;
         this.RegisterWorkflowExecutors(workflow);
 
diff --git a/dotnet/tests/Microsoft.Agents.AI.DurableTask.UnitTests/Workflows/DurableWorkflowOptionsTests.cs b/dotnet/tests/Microsoft.Agents.AI.DurableTask.UnitTests/Workflows/DurableWorkflowOptionsTests.cs
new file mode 100644
index 0000000..3a28eda
--- /dev/null
+++ b/dotnet/tests/Microsoft.Agents.AI.DurableTask.UnitTests/Workflows/DurableWorkflowOptionsTests.cs
@@ -0,0 +1,121 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using Microsoft.Agents.AI.DurableTask.Workflows;
+using Microsoft.Agents.AI.Workflows;
+
+namespace Microsoft.Agents.AI.DurableTask.UnitTests.Workflows;
+
+/// <summary>
+/// Tests for workflow registration on <see cref="DurableWorkflowOptions"/>.
+/// </summary>
+public sealed class DurableWorkflowOptionsTests
+{
+    [Fact]
+    public void AddWorkflow_ThrowsWhenDifferentWorkflowUsesRegisteredName()
+    {
+        // Arrange
+        DurableWorkflowOptions options = new DurableOptions().Workflows;
+        Workflow first = CreateWorkflow("OrderPipeline", "StepA");
+        Workflow second = CreateWorkflow("OrderPipeline", "StepB");
+
+        options.AddWorkflow(first);
+
+        // Act
+        ArgumentException ex = Assert.Throws<ArgumentException>(() => options.AddWorkflow(second));
+
+        // Assert
+        Assert.Contains("has already been registered", ex.Message, StringComparison.Ordinal);
+        Assert.Same(first, options.Workflows["OrderPipeline"]);
+    }
+
+    [Fact]
+    public void AddWorkflow_ThrowsWhenNameDiffersOnlyByCase()
+    {
+        // Arrange - workflow names are compared case-insensitively because they map to orchestration names.
+        DurableWorkflowOptions options = new DurableOptions().Workflows;
+        Workflow first = CreateWorkflow("OrderPipeline", "StepA");
+        options.AddWorkflow(first);
+
+        // Act
+        Assert.Throws<ArgumentException>(() => options.AddWorkflow(CreateWorkflow("orderpipeline", "StepB")));
+
+        // Assert - the original registration is left untouched.
+        Assert.Same(first, Assert.Single(options.Workflows).Value);
+    }
+
+    [Fact]
+    public void AddWorkflow_IsIdempotentForSameInstance()
+    {
+        // Arrange
+        DurableWorkflowOptions options = new DurableOptions().Workflows;
+        Workflow workflow = CreateWorkflow("OrderPipeline", "Step");
+
+        // Act
+        options.AddWorkflow(workflow);
+        options.AddWorkflow(workflow);
+
+        // Assert
+        Assert.Single(options.Workflows);
+        Assert.Same(workflow, options.Workflows["OrderPipeline"]);
+    }
+
+    [Fact]
+    public void AddWorkflow_AllowsSubWorkflowThatIsAlsoRegisteredExplicitly()
+    {
+        // Arrange - registering a parent and its sub-workflow explicitly means the recursive registration
+        // walk re-adds the same sub-workflow instance.
+        DurableWorkflowOptions options = new DurableOptions().Workflows;
+        Workflow subWorkflow = CreateWorkflow("SharedSub", "SubStep");
+        Workflow parent = CreateParentWorkflow("Parent", "ParentStep", subWorkflow, "Sub");
+
+        // Act
+        options.AddWorkflow(subWorkflow);
+        options.AddWorkflow(parent);
+        AddSubWorkflows(options, parent);
+        AddSubWorkflows(options, parent);
+
+        // Assert
+        Assert.Equal(2, options.Workflows.Count);
+        Assert.Same(subWorkflow, options.Workflows["SharedSub"]);
+    }
+
+    [Fact]
+    public void AddWorkflow_ThrowsWhenWorkflowIsNullOrUnnamed()
+    {
+        DurableWorkflowOptions options = new DurableOptions().Workflows;
+
+        Assert.Throws<ArgumentNullException>(() => options.AddWorkflow(null!));
+        Assert.Throws<ArgumentException>(() => options.AddWorkflow(
+            new WorkflowBuilder(new FunctionExecutor<string>("Step", (_, _, _) => default)).Build()));
+    }
+
+    private static void AddSubWorkflows(DurableWorkflowOptions options, Workflow workflow)
+    {
+        foreach (SubworkflowBinding binding in workflow.ReflectExecutors()
+            .Select(e => e.Value)
+            .OfType<SubworkflowBinding>())
+        {
+            options.AddWorkflow(binding.WorkflowInstance);
+        }
+    }
+
+    private static Workflow CreateWorkflow(string workflowName, string executorName) =>
+        new WorkflowBuilder(new FunctionExecutor<string>(executorName, (_, _, _) => default))
+            .WithName(workflowName)
+            .Build();
+
+    private static Workflow CreateParentWorkflow(
+        string workflowName,
+        string executorName,
+        Workflow subWorkflow,
+        string subWorkflowExecutorName)
+    {
+        FunctionExecutor<string> start = new(executorName, (_, _, _) => default);
+        ExecutorBinding subWorkflowExecutor = subWorkflow.BindAsExecutor(subWorkflowExecutorName);
+
+        return new WorkflowBuilder(start)
+            .WithName(workflowName)
+            .AddEdge(start, subWorkflowExecutor)
+            .Build();
+    }
+}