diff --git plugins/ActivityPub/ActivityPubPlugin.php plugins/ActivityPub/ActivityPubPlugin.php
index 96e26a7ac2..c27938be27 100644
--- plugins/ActivityPub/ActivityPubPlugin.php
+++ plugins/ActivityPub/ActivityPubPlugin.php
@@ -97,9 +97,17 @@ class ActivityPubPlugin extends Plugin
             // Look for a local notice (unfortunately GNU social doesn't
             // provide this functionality natively)
             try {
-                $candidate = Notice::getByID((int)substr($url, (strlen(Activitypub_notice::note_uri(0))-1)));
+                $id = (int)substr($url, (strlen(Activitypub_notice::note_uri(0))-1));
+                if ($id <= 0) {
+                    // Substr failed (url starts with 'tag:')
+                    // Avoid duplication
+                    $id = (int)basename($url);
+                }
+                $candidate = Notice::getByID($id);
                 if (Activitypub_notice::note_uri($candidate->getID()) === $url) { // Sanity check
                     return $candidate;
+                } elseif ($candidate->getLocalUrl() === $url) { // Sanity check (url starts with 'tag:')
+                    return $candidate;
                 } else {
                     common_debug('ActivityPubPlugin Notice Grabber: '.$candidate->getUrl(). ' is different of '.$url);
                 }
@@ -127,6 +135,12 @@ class ActivityPubPlugin extends Plugin
                     throw new Exception("A notice can't be created without an actor.");
                 }
                 if (parse_url($acclaimed_actor_profile->getUri(), PHP_URL_HOST) == parse_url($object['id'], PHP_URL_HOST)) {
+                    try {
+                        // Keep conversation tree
+                        return Notice::getByUri($object['id']);
+                    } catch (Exception $e) {
+                        // NOOP
+                    }
                     return Activitypub_notice::create_notice($object, $acclaimed_actor_profile);
                 } else {
                     throw new Exception("The acclaimed actor didn't create this note.");
diff --git plugins/ActivityPub/lib/models/Activitypub_notice.php plugins/ActivityPub/lib/models/Activitypub_notice.php
index 908b804ae0..14b4c3eae2 100644
--- plugins/ActivityPub/lib/models/Activitypub_notice.php
+++ plugins/ActivityPub/lib/models/Activitypub_notice.php
@@ -347,8 +347,15 @@ class Activitypub_notice
     public static function getUri(Notice $notice): string
     {
         if ($notice->isLocal()) {
-            return common_local_url('apNotice', ['id' => $notice->getID()]);
+            // 'common_local_url' returns '/activity/xxx' (has problem for conversation tree)
+            // 'note_url' return '/object/note/xxx'
+            return self::note_uri($notice->getID());
         } else {
+            // use 'uri' instead of 'url' (avoid duplication)
+            $uri = $notice->getUri();
+            if (common_valid_http_url($uri)) {
+                return $uri;
+            }
             return $notice->getUrl();
         }
     }