diff --git a/slices/micropub/repos/post_repo.rb b/slices/micropub/repos/post_repo.rb
index 6212667..7296541 100644
--- a/slices/micropub/repos/post_repo.rb
+++ b/slices/micropub/repos/post_repo.rb
@@ -53,6 +53,12 @@ module Micropub
           .one
       end
 
+      def url_exists?(url)
+        !!posts
+          .where(url: url)
+          .one
+      end
+
       def find!(id)
         posts
           .by_pk(id)
diff --git a/slices/micropub/validation/posts/bookmark_contract.rb b/slices/micropub/validation/posts/bookmark_contract.rb
index 0b1eb57..c8afe4d 100644
--- a/slices/micropub/validation/posts/bookmark_contract.rb
+++ b/slices/micropub/validation/posts/bookmark_contract.rb
@@ -2,6 +2,9 @@ module Micropub
   module Validation
     module Posts
       class BookmarkContract < Dry::Validation::Contract
+
+        include Deps["repos.post_repo"]
+
         params do
           required(:name).filled(:string)
           required(:content).maybe(:string)
@@ -14,6 +17,10 @@ module Micropub
           required(:photos).array(:hash)
           required(:cache).filled(:bool)
         end
+
+        rule(:url) do
+          key.failure("URL already bookmarked") if post_repo.url_exists?(values[:url])
+        end
       end
     end
   end
diff --git a/spec/requests/create_post_spec.rb b/spec/requests/create_post_spec.rb
index 34c40d9..9a66952 100644
--- a/spec/requests/create_post_spec.rb
+++ b/spec/requests/create_post_spec.rb
@@ -176,6 +176,26 @@ RSpec.describe "Post creation", :db, :requests do
       expect(post_repo.bookmark_listing.count).to eq 1
       expect(post_repo.bookmark_listing.last.cached_content).to_not eq nil
     end
+
+    it "validates an already bookmarked URL" do
+      params = {
+        h: "entry",
+        "bookmark-of": "http://example.com",
+        name: "Name",
+        content: "Content of the post",
+        cache: "true"
+      }
+
+      post "/micropub", params
+
+      expect(last_response).to be_successful
+      expect(post_repo.bookmark_listing.count).to eq 1
+      expect(post_repo.bookmark_listing.last.cached_content).to_not eq nil
+      
+      post "/micropub", params
+      expect(last_response).to_not be_successful
+      expect(post_repo.bookmark_listing.count).to eq 1
+    end
   end
 
   context "checkins" do