diff --git a/internal/memory/recalleval/recalleval.go b/internal/memory/recalleval/recalleval.go
index 1691b49..e1be017 100644
--- a/internal/memory/recalleval/recalleval.go
+++ b/internal/memory/recalleval/recalleval.go
@@ -90,9 +90,44 @@ func Load() (Fixture, error) {
 	if len(f.Cases) == 0 {
 		return Fixture{}, fmt.Errorf("fixture has no cases")
 	}
+	if err := checkIDs(f); err != nil {
+		return Fixture{}, err
+	}
 	return f, nil
 }
 
+// checkIDs refuses a fixture where a case note and a filler note share an id.
+//
+// Every case is scored over its own notes plus the whole filler set, and the
+// two stores disagree about what a repeated id means: the sqlite store upserts
+// on it, the in-memory store appends. So one collision makes a case score
+// differently on the two backends, and it reads as an embedder or gate
+// difference, which is the one thing this harness exists to measure (Vikunja
+// #386). It was dodged once by hand during #373 by renaming two ids.
+//
+// Checked in Load rather than in the test, so every caller of the fixture is
+// covered and not only the one that remembers to look.
+func checkIDs(f Fixture) error {
+	filler := make(map[string]bool, len(f.Filler))
+	for _, n := range f.Filler {
+		if n.ID == "" {
+			return fmt.Errorf("filler note with an empty id")
+		}
+		if filler[n.ID] {
+			return fmt.Errorf("duplicate filler note id %q", n.ID)
+		}
+		filler[n.ID] = true
+	}
+	for _, c := range f.Cases {
+		for _, n := range c.Notes {
+			if filler[n.ID] {
+				return fmt.Errorf("case %s: note id %q collides with a filler note", c.ID, n.ID)
+			}
+		}
+	}
+	return nil
+}
+
 // NewStore builds an empty store for one case, plus a function to release it.
 // A factory rather than a store because every case needs a clean index — notes
 // from case A must not be visible to case B's query.
diff --git a/internal/memory/recalleval/recalleval_test.go b/internal/memory/recalleval/recalleval_test.go
index e9cea1c..f8b324e 100644
--- a/internal/memory/recalleval/recalleval_test.go
+++ b/internal/memory/recalleval/recalleval_test.go
@@ -337,3 +337,28 @@ func marginSweep(t *testing.T, emb router.Embedder, f Fixture) string {
 	}
 	return b.String()
 }
+
+// TestFillerIDCollisionIsRefused — the guard that keeps a fixture edit from
+// looking like a backend difference (Vikunja #386).
+func TestFillerIDCollisionIsRefused(t *testing.T) {
+	f := Fixture{
+		SchemaVersion: SchemaVersion,
+		Cases:         []Case{{ID: "ru-001", Notes: []StoredNote{{ID: "f1", Text: "..."}}}},
+		Filler:        []StoredNote{{ID: "f1", Text: "..."}},
+	}
+	if err := checkIDs(f); err == nil {
+		t.Fatal("a case note reusing a filler id must be refused")
+	}
+	f.Filler = append(f.Filler, StoredNote{ID: "f1", Text: "..."})
+	if err := checkIDs(Fixture{SchemaVersion: SchemaVersion, Filler: f.Filler}); err == nil {
+		t.Fatal("a duplicate filler id must be refused")
+	}
+	ok := Fixture{
+		SchemaVersion: SchemaVersion,
+		Cases:         []Case{{ID: "ru-001", Notes: []StoredNote{{ID: "n1", Text: "..."}}}},
+		Filler:        []StoredNote{{ID: "f1", Text: "..."}},
+	}
+	if err := checkIDs(ok); err != nil {
+		t.Fatalf("a clean fixture must pass: %v", err)
+	}
+}