diff --git a/app/models/project.rb b/app/models/project.rb
index 178d70757a1..67593c9b2fe 100644
--- a/app/models/project.rb
+++ b/app/models/project.rb
@@ -85,8 +85,7 @@ class Project < ActiveRecord::Base
   after_create :create_project_feature, unless: :project_feature
 
   after_create -> { SiteStatistic.track(STATISTICS_ATTRIBUTE) }
-  before_destroy ->(project) { project.project_feature.untrack_statistics_for_deletion! }
-  after_destroy -> { SiteStatistic.untrack(STATISTICS_ATTRIBUTE) }
+  before_destroy :untrack_site_statistics
 
   after_create :create_ci_cd_settings,
     unless: :ci_cd_settings,
@@ -2099,6 +2098,11 @@ class Project < ActiveRecord::Base
     Gitlab::PagesTransfer.new.rename_project(path_before, self.path, namespace.full_path)
   end
 
+  def untrack_site_statistics
+    SiteStatistic.untrack(STATISTICS_ATTRIBUTE)
+    self.project_feature.untrack_statistics_for_deletion!
+  end
+
   def execute_rename_repository_hooks!(full_path_before)
     # When we import a project overwriting the original project, there
     # is a move operation. In that case we don't want to send the instructions.
diff --git a/app/services/groups/destroy_service.rb b/app/services/groups/destroy_service.rb
index 12aeba4af71..93d84bd8a9c 100644
--- a/app/services/groups/destroy_service.rb
+++ b/app/services/groups/destroy_service.rb
@@ -12,12 +12,15 @@ module Groups
     def execute
       group.prepare_for_destroy
 
-      group.projects.each do |project|
+      group.projects.includes(:project_feature).each do |project|
         # Execute the destruction of the models immediately to ensure atomic cleanup.
         success = ::Projects::DestroyService.new(project, current_user).execute
         raise DestroyError, "Project #{project.id} can't be deleted" unless success
       end
 
+      # reload the relation to prevent triggering destroy hooks on the projects again
+      group.projects.reload
+
       group.children.each do |group|
         # This needs to be synchronous since the namespace gets destroyed below
         DestroyService.new(group, current_user).execute
diff --git a/changelogs/unreleased/50441-high-number-of-statement-timeouts-in-groupdestroyworker-due-to-sitestatistics.yml b/changelogs/unreleased/50441-high-number-of-statement-timeouts-in-groupdestroyworker-due-to-sitestatistics.yml
new file mode 100644
index 00000000000..3e360f8d6bb
--- /dev/null
+++ b/changelogs/unreleased/50441-high-number-of-statement-timeouts-in-groupdestroyworker-due-to-sitestatistics.yml
@@ -0,0 +1,5 @@
+---
+title: Removing a group no longer triggers hooks for project deletion twice
+merge_request: 21366
+author:
+type: fixed
diff --git a/db/post_migrate/20180826111825_recalculate_site_statistics.rb b/db/post_migrate/20180826111825_recalculate_site_statistics.rb
new file mode 100644
index 00000000000..741035a444f
--- /dev/null
+++ b/db/post_migrate/20180826111825_recalculate_site_statistics.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+class RecalculateSiteStatistics < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    transaction do
+      execute('SET LOCAL statement_timeout TO 0') if Gitlab::Database.postgresql? # see https://gitlab.com/gitlab-org/gitlab-ce/issues/48967
+
+      execute("UPDATE site_statistics SET repositories_count = (SELECT COUNT(*) FROM projects)")
+    end
+
+    transaction do
+      execute('SET LOCAL statement_timeout TO 0') if Gitlab::Database.postgresql? # see https://gitlab.com/gitlab-org/gitlab-ce/issues/48967
+
+      execute("UPDATE site_statistics SET wikis_count = (SELECT COUNT(*) FROM project_features WHERE wiki_access_level != 0)")
+    end
+  end
+
+  def down
+    # No downside in keeping the counter up-to-date
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 380d4e49ddf..cb8f90efded 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180816193530) do
+ActiveRecord::Schema.define(version: 20180826111825) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
diff --git a/spec/services/groups/destroy_service_spec.rb b/spec/services/groups/destroy_service_spec.rb
index d80d0f5a8a8..97a88b5d697 100644
--- a/spec/services/groups/destroy_service_spec.rb
+++ b/spec/services/groups/destroy_service_spec.rb
@@ -35,6 +35,14 @@ describe Groups::DestroyService do
       it { expect(NotificationSetting.unscoped.all).not_to include(notification_setting) }
     end
 
+    context 'site statistics' do
+      it 'doesnt trigger project deletion hooks twice' do
+        expect_any_instance_of(Project).to receive(:untrack_site_statistics).once
+
+        destroy_group(group, user, async)
+      end
+    end
+
     context 'mattermost team' do
       let!(:chat_team) { create(:chat_team, namespace: group) }