diff --git a/src/invidious/routes/api/v1/authenticated.cr b/src/invidious/routes/api/v1/authenticated.cr
index a35d2f2b..c469eb6c 100644
--- a/src/invidious/routes/api/v1/authenticated.cr
+++ b/src/invidious/routes/api/v1/authenticated.cr
@@ -313,6 +313,11 @@ module Invidious::Routes::API::V1::Authenticated
       return error_json(403, "Invalid videoId")
     end
 
+    # Prevent duplicate videos in the same playlist
+    if Invidious::Database::PlaylistVideos.select_index(plid, video_id)
+      return error_json(409, "Video already exists in this playlist")
+    end
+
     begin
       video = get_video(video_id)
     rescue ex : NotFoundException
diff --git a/src/invidious/routes/playlists.cr b/src/invidious/routes/playlists.cr
index 56e529b2..7fd0caf3 100644
--- a/src/invidious/routes/playlists.cr
+++ b/src/invidious/routes/playlists.cr
@@ -330,6 +330,15 @@ module Invidious::Routes::Playlists
 
       video_id = env.params.query["video_id"]
 
+      # Prevent duplicate videos in the same playlist
+      if Invidious::Database::PlaylistVideos.select_index(playlist_id, video_id)
+        if redirect
+          return env.redirect referer
+        else
+          return error_json(409, "Video already exists in this playlist")
+        end
+      end
+
       begin
         video = get_video(video_id)
       rescue ex : NotFoundException