diff --git a/pub/fed.go b/pub/fed.go
index 51dd5dd..215c592 100644
--- a/pub/fed.go
+++ b/pub/fed.go
@@ -205,15 +205,18 @@ func (f *federator) PostInbox(c context.Context, w http.ResponseWriter, r *http.
 	if err = f.FederateAPI.Unblocked(c, iris); err != nil {
 		return true, err
 	}
-	if err = f.getPostInboxResolver(c, r.URL).Deserialize(m); err != nil {
+	if err := f.addToInboxIfNew(c, r, m, func() error {
+		if err = f.getPostInboxResolver(c, r.URL).Deserialize(m); err != nil {
+			return err
+		}
+		return nil
+	}); err != nil {
 		if err == errObjectRequired || err == errTargetRequired {
 			w.WriteHeader(http.StatusBadRequest)
 			return true, nil
+		} else {
+			return true, err
 		}
-		return true, err
-	}
-	if err := f.addToInbox(c, r, m); err != nil {
-		return true, err
 	}
 	if err := f.inboxForwarding(c, m); err != nil {
 		return true, err
diff --git a/pub/fed_test.go b/pub/fed_test.go
index 15818a0..8e9ea91 100644
--- a/pub/fed_test.go
+++ b/pub/fed_test.go
@@ -1989,10 +1989,14 @@ func TestPostInbox_RequiresObject(t *testing.T) {
 			},
 		},
 	}
-	_, _, fedApp, _, _, _, _, p := NewPubberTest(t)
+	app, _, fedApp, _, _, _, _, p := NewPubberTest(t)
 	fedApp.unblocked = func(c context.Context, actorIRIs []*url.URL) error {
 		return nil
 	}
+	app.MockFederateApp.getInbox = func(c context.Context, r *http.Request, rw RWType) (vocab.OrderedCollectionType, error) {
+		inbox := &vocab.OrderedCollection{}
+		return inbox, nil
+	}
 	for _, test := range tests {
 		t.Logf("Running table test case %q", test.name)
 		resp := httptest.NewRecorder()
@@ -2036,10 +2040,14 @@ func TestPostInbox_RequiresTarget(t *testing.T) {
 			},
 		},
 	}
-	_, _, fedApp, _, _, _, _, p := NewPubberTest(t)
+	app, _, fedApp, _, _, _, _, p := NewPubberTest(t)
 	fedApp.unblocked = func(c context.Context, actorIRIs []*url.URL) error {
 		return nil
 	}
+	app.MockFederateApp.getInbox = func(c context.Context, r *http.Request, rw RWType) (vocab.OrderedCollectionType, error) {
+		inbox := &vocab.OrderedCollection{}
+		return inbox, nil
+	}
 	for _, test := range tests {
 		t.Logf("Running table test case %q", test.name)
 		resp := httptest.NewRecorder()
@@ -2080,8 +2088,8 @@ func TestPostInbox_DoesNotAddToInboxIfDuplicate(t *testing.T) {
 		t.Fatal(err)
 	} else if !handled {
 		t.Fatalf("expected handled, got !handled")
-	} else if gotSet != 1 {
-		t.Fatalf("expected %d, got %d", 1, gotSet)
+	} else if gotSet != 0 {
+		t.Fatalf("expected %d, got %d", 0, gotSet)
 	}
 }
 
diff --git a/pub/internal.go b/pub/internal.go
index 4da0c6b..9fc1f22 100644
--- a/pub/internal.go
+++ b/pub/internal.go
@@ -1763,7 +1763,7 @@ func (f *federator) addToOutbox(c context.Context, r *http.Request, m map[string
 	return f.App.Set(c, outbox)
 }
 
-func (f *federator) addToInbox(c context.Context, r *http.Request, m map[string]interface{}) error {
+func (f *federator) addToInboxIfNew(c context.Context, r *http.Request, m map[string]interface{}, callback func() error) error {
 	inbox, err := f.App.GetInbox(c, r, ReadWrite)
 	if err != nil {
 		return err
@@ -1780,6 +1780,9 @@ func (f *federator) addToInbox(c context.Context, r *http.Request, m map[string]
 		return fmt.Errorf("activity missing id")
 	}
 	if !iriSet[activity.GetId().String()] {
+		if err := callback(); err != nil {
+			return err
+		}
 		inbox.PrependOrderedItemsIRI(activity.GetId())
 		return f.App.Set(c, inbox)
 	}