fix step duplication

src/Runner.Worker/Dap/DapDebugSession.cs

@@ -1770,8 +1770,8 @@ namespace GitHub.Runner.Worker.Dap
             }
 
             // Reset the step manipulator to match the restored state
-            // It will be re-initialized when the restored step starts
             _stepManipulator?.ClearChanges();
+            _stepManipulator?.TrimCompletedSteps(checkpointIndex);
 
             // Store restored checkpoint for StepsRunner to consume
             _restoredCheckpoint = checkpoint;

src/Runner.Worker/Dap/StepCommands/StepManipulator.cs

@@ -114,6 +114,13 @@ namespace GitHub.Runner.Worker.Dap.StepCommands
         /// </summary>
         void ClearChanges();
 
+        /// <summary>
+        /// Trims the completed steps list to the specified count.
+        /// Used when restoring a checkpoint to sync state with the debug session.
+        /// </summary>
+        /// <param name="count">The number of completed steps to keep.</param>
+        void TrimCompletedSteps(int count);
+
         /// <summary>
         /// Checks if a step with the given ID already exists.
         /// </summary>
@@ -576,6 +583,17 @@ namespace GitHub.Runner.Worker.Dap.StepCommands
             _originalSteps = null;
         }
 
+        /// <inheritdoc/>
+        public void TrimCompletedSteps(int count)
+        {
+            var originalCount = _completedSteps.Count;
+            while (_completedSteps.Count > count)
+            {
+                _completedSteps.RemoveAt(_completedSteps.Count - 1);
+            }
+            Trace.Info($"Trimmed completed steps from {originalCount} to {_completedSteps.Count}");
+        }
+
         /// <inheritdoc/>
         public bool HasStepWithId(string id)
         {