diff --git a/modules/backlogs/spec/models/friendly_id/slug_spec.rb b/modules/backlogs/spec/models/friendly_id/slug_spec.rb
new file mode 100644
index 00000000000..d9c48dad3a2
--- /dev/null
+++ b/modules/backlogs/spec/models/friendly_id/slug_spec.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+require "spec_helper"
+
+RSpec.describe FriendlyId::Slug do
+  describe "friendly_id_slugs(slug, sluggable_type, scope) unique constraints" do
+    it "ensures uniqueness with null scope" do
+      p = create(:project)
+      expect do
+        described_class.create!(sluggable: p, slug: "my-app", scope: nil)
+        described_class.create!(sluggable: p, slug: "my-app", scope: nil)
+      end.to raise_error(
+        ActiveRecord::RecordNotUnique,
+        /PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_friendly_id_slugs_on_slug_and_sluggable_type_and_scope"\n/ # rubocop:disable Layout/LineLength
+      )
+    end
+  end
+end
diff --git a/spec/models/project_spec.rb b/spec/models/project_spec.rb
index 254b258af84..fbebf0d552e 100644
--- a/spec/models/project_spec.rb
+++ b/spec/models/project_spec.rb
@@ -606,4 +606,16 @@ RSpec.describe Project do
       end
     end
   end
+
+  describe "projects.identifier unique database constraint" do
+    let!(:other_project) { create(:project, identifier: "my-app") }
+    let(:project) { build(:project, identifier: "my-app") }
+
+    it "ensures uniqueness with disabled validation" do
+      expect { project.save!(validate: false) }.to raise_error(
+        ActiveRecord::RecordNotUnique,
+        /PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_projects_on_lower_identifier"\n/
+      )
+    end
+  end
 end