fix: raise ValueError on duplicate subtask tags in reorder_subtasks

cli/decompose/decompose.py

@@ -62,8 +62,18 @@ def reorder_subtasks(
         come before dependents, with numbering prefixes updated.
 
     Raises:
+        ValueError: If duplicate subtask tags are detected (case-insensitive).
         ValueError: If a circular dependency is detected.
     """
+    seen: set[str] = set()
+    for subtask in subtasks:
+        tag = subtask["tag"].lower()
+        if tag in seen:
+            raise ValueError(
+                f'Duplicate subtask tag "{tag}". Tags must be unique (case-insensitive).'
+            )
+        seen.add(tag)
+
     subtask_map = {subtask["tag"].lower(): subtask for subtask in subtasks}
 
     graph = {}

test/cli/test_decompose_unit.py

@@ -62,6 +62,18 @@ def test_reorder_circular_raises():
         reorder_subtasks(subtasks)
 
 
+def test_reorder_duplicate_tag_raises():
+    subtasks = [_subtask("a", "1. Task A"), _subtask("a", "2. Also A")]
+    with pytest.raises(ValueError, match="Duplicate subtask tag"):
+        reorder_subtasks(subtasks)
+
+
+def test_reorder_duplicate_tag_case_insensitive_raises():
+    subtasks = [_subtask("Step_A", "1. Task A"), _subtask("step_a", "2. Also A")]
+    with pytest.raises(ValueError, match="Duplicate subtask tag"):
+        reorder_subtasks(subtasks)
+
+
 def test_reorder_renumbers_subtasks():
     subtasks = [
         _subtask("b", "2. Task B", depends_on=["a"]),