diff --git a/modules/bouncer/README.md b/modules/bouncer/README.md
index 01ebe36..0973255 100644
--- a/modules/bouncer/README.md
+++ b/modules/bouncer/README.md
@@ -78,6 +78,7 @@ func Login(secret string) (string, error) {
 | `bouncer.Registry` | Named guard registry; `bouncer.NewRegistry` creates one. |
 | `bouncer.Registry.Register` | Registers a guard under a name on behalf of a plugin; duplicate names fail. |
 | `bouncer.Registry.Middleware` | Returns HTTP middleware for a registered guard; unknown names fail. |
+| `bouncer.Registry.Owner` | The plugin ID that registered a named guard. |
 | `bouncer.NewJWTGuard` | Frontend JWT guard reading the bearer header and optional cookies. |
 | `bouncer.NewBackendJWTGuard` | Admin JWT guard that requires the backend audience. |
 | `bouncer.Middleware` | Standalone middleware that validates a bearer token and loads the user. |
diff --git a/modules/bouncer/registry.go b/modules/bouncer/registry.go
index 116b5a7..1304b7e 100644
--- a/modules/bouncer/registry.go
+++ b/modules/bouncer/registry.go
@@ -54,6 +54,16 @@ func (reg *Registry) Register(pluginID, name string, g any) error {
 	return nil
 }
 
+// Owner returns the plugin ID that registered the guard name. The second
+// result is false when no guard has that name.
+func (reg *Registry) Owner(name string) (string, bool) {
+	if reg == nil {
+		return "", false
+	}
+	ng, ok := reg.guards[name]
+	return ng.pluginID, ok
+}
+
 // Middleware derives an http middleware from a registered guard. Unknown
 // names fail (fail boot, mirrors surf.RegisterMiddleware's contract).
 // On Authenticate/AuthenticateCredential success: WithUser (+WithCredential
diff --git a/modules/bouncer/registry_test.go b/modules/bouncer/registry_test.go
index 4ad8c1d..ce4851c 100644
--- a/modules/bouncer/registry_test.go
+++ b/modules/bouncer/registry_test.go
@@ -289,3 +289,20 @@ func TestRegisterAcceptsValidGuards(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestRegistryOwner(t *testing.T) {
+	reg := NewRegistry()
+	if _, ok := reg.Owner("backend"); ok {
+		t.Fatal("an unregistered guard reported an owner")
+	}
+	if err := reg.Register("acme.owner", "backend", NewBackendJWTGuard("test-secret-for-registry-owner", nil, nil, nil)); err != nil {
+		t.Fatal(err)
+	}
+	if owner, ok := reg.Owner("backend"); !ok || owner != "acme.owner" {
+		t.Fatalf("Owner = %q, %v; want acme.owner", owner, ok)
+	}
+	var nilReg *Registry
+	if _, ok := nilReg.Owner("backend"); ok {
+		t.Fatal("a nil registry reported an owner")
+	}
+}
diff --git a/modules/cabana/backend_guard_collision_test.go b/modules/cabana/backend_guard_collision_test.go
index bf15cbe..de2a7b6 100644
--- a/modules/cabana/backend_guard_collision_test.go
+++ b/modules/cabana/backend_guard_collision_test.go
@@ -39,4 +39,8 @@ func TestActivateRefusesAForeignBackendGuard(t *testing.T) {
 	if _, err := cabana.Activate(clean, []party.Plugin{demoPlugin{fsys: demoFS()}}); err != nil {
 		t.Fatalf("Activate without a conflicting guard: %v", err)
 	}
+	// A second assembly on the same application finds cabana's own guard.
+	if _, err := cabana.Activate(clean, []party.Plugin{demoPlugin{fsys: demoFS()}}); err != nil {
+		t.Fatalf("second Activate on one application: %v", err)
+	}
 }
diff --git a/modules/cabana/http.go b/modules/cabana/http.go
index 255c7c2..8ae2629 100644
--- a/modules/cabana/http.go
+++ b/modules/cabana/http.go
@@ -117,8 +117,14 @@ func Activate(app *backpack.App, plugins []party.Plugin) (*Routes, error) {
 	// provider), so cabana always registers its own and never mounts the API
 	// behind a guard another plugin already put under the name "backend": a
 	// taken name fails boot instead of silently replacing admin authentication.
-	if err := guards.Register("summercms.cabana", "backend", guard); err != nil {
-		return nil, fmt.Errorf("cabana: backend guard: %w", err)
+	// Activating twice on one application (a test assembling two handlers)
+	// finds cabana's own guard and keeps it.
+	if owner, taken := guards.Owner("backend"); !taken {
+		if err := guards.Register("summercms.cabana", "backend", guard); err != nil {
+			return nil, fmt.Errorf("cabana: backend guard: %w", err)
+		}
+	} else if owner != "summercms.cabana" {
+		return nil, fmt.Errorf("cabana: backend guard: guard %q is already registered by %s; the admin API needs its own audience-checking guard under that name", "backend", owner)
 	}
 	mw, err := guards.Middleware("backend")
 	if err != nil {