Fix blocking subdomains of an already-blocked domain (#26392)

app/controllers/admin/domain_blocks_controller.rb

@@ -40,7 +40,7 @@ module Admin
       end
 
       # Allow transparently upgrading a domain block
-      if existing_domain_block.present?
+      if existing_domain_block.present? && existing_domain_block.domain == TagManager.instance.normalize_domain(@domain_block.domain.strip)
         @domain_block = existing_domain_block
         @domain_block.assign_attributes(resource_params)
       end