fix(compile): drop same-batch duplicate page titles as collisions

CHANGELOG.md

@@ -6,6 +6,14 @@ All notable changes to vouch are documented here. Format follows
 
 ## [Unreleased]
 
+### Fixed
+- `compile_kb` no longer lets two same-titled (or same-slugifying) page
+  drafts in a single LLM response both survive validation. The second
+  draft is now dropped as a collision, same as if it collided with a
+  pre-existing page — previously, approving both proposals in order would
+  silently overwrite the first approved page via the `update_page`
+  vault-edit path, with no error or diff surfaced to the reviewer.
+
 ## [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 — a later
+        # draft in the same batch colliding with this one must be dropped,
+        # same as if it collided with a pre-existing page (see
+        # _draft_problem's collision-guard comment above).
+        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

@@ -301,6 +301,26 @@ def test_collision_with_existing_page_dropped(
     assert store.get_page("deploy-workflow").body == before
 
 
+def test_collision_within_same_batch_dropped(
+    store: KBStore, tmp_path: Path,
+) -> None:
+    """Two same-titled drafts in one compile_kb() call must not both survive —
+    approving the first creates the page, approving the second would silently
+    overwrite it via the update_page vault-edit path (see #439)."""
+    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 [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_collision_with_pending_proposal_dropped(
     store: KBStore, tmp_path: Path,
 ) -> None: