fix(compile): drop same-batch duplicate page titles before approval

CHANGELOG.md

@@ -6,6 +6,12 @@ All notable changes to vouch are documented here. Format follows
 
 ## [Unreleased]
 
+### Fixed
+- `compile_kb()` no longer lets two same-titled drafts in one LLM response
+  both survive the collision guard — the second draft is now dropped instead
+  of silently overwriting the first-approved page via the PAGE `update_page`
+  fallback in `approve()`. (#439)
+
 ## [1.2.2] — 2026-07-07
 
 ### Packaging

src/vouch/compile.py

@@ -343,6 +343,12 @@ def compile_kb(
             report.dropped.append({"title": title, "reason": problem})
             continue
         survivors.append((draft, title))
+        # fold the accepted title into taken_names immediately — otherwise a
+        # second draft later in the same batch with a colliding title passes
+        # the collision guard too, and approving both silently overwrites the
+        # first-approved page via the PAGE update_page fallback in approve().
+        taken_names.add(title.lower())
+        taken_names.add(_slugify(title))
 
     # phase 2: wikilinks resolve against existing pages + the *surviving*
     # batch, to a fixpoint — dropping a draft may dangle a link in another,

tests/test_compile.py

@@ -317,6 +317,26 @@ def test_collision_with_pending_proposal_dropped(
     assert "pending review" in second.dropped[0]["reason"]
 
 
+def test_collision_within_same_batch_second_draft_dropped(
+    store: KBStore, tmp_path: Path,
+) -> None:
+    """two drafts in one LLM response with the same title must not both
+    survive validation — approving both would silently overwrite the first
+    page via the PAGE update_page fallback in approve()."""
+    c1 = _approved_claim(store, "a fact")
+    report = compile_kb(store, config=_cfg(_stub_llm(tmp_path, [
+        {"title": "Deploy Workflow", "type": "workflow",
+         "body": f"first body [claim: {c1}]", "claims": [c1]},
+        {"title": "Deploy Workflow", "type": "workflow",
+         "body": f"SECOND BODY totally different [claim: {c1}]", "claims": [c1]},
+    ])))
+    assert len(report.proposed) == 1
+    assert "already exists" in report.dropped[0]["reason"]
+
+    approve(store, report.proposed[0]["proposal_id"], approved_by="human-B")
+    assert store.get_page("deploy-workflow").body == f"first body [claim: {c1}]"
+
+
 def test_dropped_sibling_dangles_wikilink_and_cascades(
     store: KBStore, tmp_path: Path,
 ) -> None: