fix: prevent same-batch duplicate page proposals in compile_kb (#439)

CHANGELOG.md

@@ -21,6 +21,14 @@ All notable changes to vouch are documented here. Format follows
 - console Dashboard view: 12-month activity calendar, last-30-days bars,
   hour-of-week heatmap, top actors and event mix, driven by `kb.activity`.
 
+### Fixed
+- `compile_kb()` could file two page proposals for the same title when the
+  LLM's batch drafted the same topic (or same slug, e.g. "Retry Policy" vs
+  "retry policy") twice — `taken_names` was only seeded from on-disk pages
+  and pending proposals, never updated as drafts were accepted within the
+  batch. Approving the second proposal would silently route through
+  `update_page()` and overwrite the first. (#439)
+
 ## [1.2.2] — 2026-07-07
 
 ### Packaging

src/vouch/compile.py

@@ -312,6 +312,12 @@ def compile_kb(
             report.dropped.append({"title": title, "reason": problem})
             continue
         survivors.append((draft, title))
+        # Fix #439: fold accepted title/slug into taken_names immediately so
+        # a later draft in the same batch cannot collide with a just-accepted
+        # one. Without this, two drafts with the same title both pass
+        # _draft_problem's collision guard and land as duplicate proposals.
+        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

@@ -417,3 +417,75 @@ def test_compile_logs_attributed_audit_event(
     events = [e for e in audit_mod.read_events(store.kb_dir)
               if e.event == "compile.run"]
     assert len(events) == 1
+
+
+def test_same_batch_duplicate_title_drops_second(
+    store: KBStore, tmp_path: Path,
+) -> None:
+    """Regression for #439: when the LLM returns two drafts with the same
+    title in a single batch, the second must be dropped as a collision rather
+    than both being filed as proposals (which would cause a duplicate-proposal
+    crash on the second approve).
+
+    Before the fix, taken_names was only seeded from already-on-disk pages
+    and pending proposals — it was never updated as drafts were accepted in
+    the phase-1 loop, so a sibling draft with the same title slipped through
+    _draft_problem's collision guard."""
+    c1 = _approved_claim(store, "a fact about retries")
+    c2 = _approved_claim(store, "a second fact about retries")
+    cmd = _stub_llm(tmp_path, [
+        {
+            "title": "Retry Policy",
+            "type": "decision",
+            "body": f"First version [claim: {c1}]",
+            "claims": [c1],
+        },
+        {
+            "title": "Retry Policy",
+            "type": "decision",
+            "body": f"Duplicate version [claim: {c2}]",
+            "claims": [c2],
+        },
+    ])
+    report = compile_kb(store, config=_cfg(cmd))
+
+    # Only the first draft should be proposed; the second is a same-batch collision.
+    assert len(report.proposed) == 1
+    assert report.proposed[0]["title"] == "Retry Policy"
+    assert len(report.dropped) == 1
+    assert report.dropped[0]["title"] == "Retry Policy"
+    assert "already exists or is pending review" in report.dropped[0]["reason"]
+
+    # Confirm only one pending proposal was filed.
+    pending = store.list_proposals(ProposalStatus.PENDING)
+    assert len(pending) == 1
+    assert pending[0].payload["title"] == "Retry Policy"
+
+
+def test_same_batch_duplicate_slug_drops_second(
+    store: KBStore, tmp_path: Path,
+) -> None:
+    """Regression for #439: titles that slugify to the same id (e.g. 'Retry
+    Policy' and 'retry-policy') must also be caught as same-batch collisions,
+    since approve() uses the slug as the on-disk page id."""
+    c1 = _approved_claim(store, "a fact about auth")
+    c2 = _approved_claim(store, "another fact about auth")
+    cmd = _stub_llm(tmp_path, [
+        {
+            "title": "Auth Flow",
+            "type": "workflow",
+            "body": f"First [claim: {c1}]",
+            "claims": [c1],
+        },
+        {
+            "title": "auth flow",  # same slug as above
+            "type": "workflow",
+            "body": f"Duplicate slug [claim: {c2}]",
+            "claims": [c2],
+        },
+    ])
+    report = compile_kb(store, config=_cfg(cmd))
+
+    assert len(report.proposed) == 1
+    assert len(report.dropped) == 1
+    assert "already exists or is pending review" in report.dropped[0]["reason"]