diff --git a/changelog.d/3888.fix b/changelog.d/3888.fix
new file mode 100644
index 000000000..886aa7b39
--- /dev/null
+++ b/changelog.d/3888.fix
@@ -0,0 +1 @@
+ForceMentionsInContent: fix double mentions for Mastodon/Misskey posts
\ No newline at end of file
diff --git a/lib/pleroma/web/activity_pub/mrf/force_mentions_in_content.ex b/lib/pleroma/web/activity_pub/mrf/force_mentions_in_content.ex
index 70224561c..5532093cb 100644
--- a/lib/pleroma/web/activity_pub/mrf/force_mentions_in_content.ex
+++ b/lib/pleroma/web/activity_pub/mrf/force_mentions_in_content.ex
@@ -1,5 +1,5 @@
 # Pleroma: A lightweight social networking server
-# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# Copyright © 2017-2023 Pleroma Authors <https://pleroma.social/>
 # SPDX-License-Identifier: AGPL-3.0-only
 
 defmodule Pleroma.Web.ActivityPub.MRF.ForceMentionsInContent do
@@ -95,11 +95,13 @@ def filter(
       |> Enum.reject(&is_nil/1)
       |> sort_replied_user(replied_to_user)
 
-    explicitly_mentioned_uris = extract_mention_uris_from_content(content)
+    explicitly_mentioned_uris =
+      extract_mention_uris_from_content(content)
+      |> MapSet.new()
 
     added_mentions =
-      Enum.reduce(mention_users, "", fn %User{ap_id: uri} = user, acc ->
-        unless uri in explicitly_mentioned_uris do
+      Enum.reduce(mention_users, "", fn %User{ap_id: ap_id, uri: uri} = user, acc ->
+        if MapSet.disjoint?(MapSet.new([ap_id, uri]), explicitly_mentioned_uris) do
           acc <> Formatter.mention_from_user(user, %{mentions_format: :compact}) <> " "
         else
           acc
diff --git a/test/pleroma/web/activity_pub/mrf/force_mentions_in_content_test.exs b/test/pleroma/web/activity_pub/mrf/force_mentions_in_content_test.exs
index b349a4bb7..811ef105c 100644
--- a/test/pleroma/web/activity_pub/mrf/force_mentions_in_content_test.exs
+++ b/test/pleroma/web/activity_pub/mrf/force_mentions_in_content_test.exs
@@ -256,4 +256,55 @@ test "works with Updates" do
               }
             }} = MRF.filter_one(ForceMentionsInContent, activity)
   end
+
+  test "don't add duplicate mentions for mastodon or misskey posts" do
+    [zero, rogerick, greg] = [
+      insert(:user,
+        ap_id: "https://pleroma.example.com/users/zero",
+        uri: "https://pleroma.example.com/users/zero",
+        nickname: "zero@pleroma.example.com",
+        local: false
+      ),
+      insert(:user,
+        ap_id: "https://misskey.example.com/users/104ab42f11",
+        uri: "https://misskey.example.com/@rogerick",
+        nickname: "rogerick@misskey.example.com",
+        local: false
+      ),
+      insert(:user,
+        ap_id: "https://mastodon.example.com/users/greg",
+        uri: "https://mastodon.example.com/@greg",
+        nickname: "greg@mastodon.example.com",
+        local: false
+      )
+    ]
+
+    {:ok, post} = CommonAPI.post(rogerick, %{status: "eugh"})
+
+    inline_mentions = [
+      "<span class=\"h-card\"><a class=\"u-url mention\" data-user=\"#{rogerick.id}\" href=\"#{rogerick.ap_id}\" rel=\"ugc\">@<span>rogerick</span></a></span>",
+      "<span class=\"h-card\"><a class=\"u-url mention\" data-user=\"#{greg.id}\" href=\"#{greg.uri}\" rel=\"ugc\">@<span>greg</span></a></span>"
+    ]
+
+    activity = %{
+      "type" => "Create",
+      "actor" => zero.ap_id,
+      "object" => %{
+        "type" => "Note",
+        "actor" => zero.ap_id,
+        "content" => "#{Enum.at(inline_mentions, 0)} #{Enum.at(inline_mentions, 1)} erm",
+        "to" => [
+          rogerick.ap_id,
+          greg.ap_id,
+          Constants.as_public()
+        ],
+        "inReplyTo" => Object.normalize(post).data["id"]
+      }
+    }
+
+    {:ok, %{"object" => %{"content" => filtered}}} = ForceMentionsInContent.filter(activity)
+
+    assert filtered ==
+             "#{Enum.at(inline_mentions, 0)} #{Enum.at(inline_mentions, 1)} erm"
+  end
 end