Fix error when a MX is shared across blocked domains (#17650)

app/controllers/admin/email_domain_blocks_controller.rb

@@ -38,6 +38,8 @@ module Admin
           log_action :create, @email_domain_block
 
           (@email_domain_block.other_domains || []).uniq.each do |domain|
+            next if EmailDomainBlock.where(domain: domain).exists?
+
             other_email_domain_block = EmailDomainBlock.create!(domain: domain, parent: @email_domain_block)
             log_action :create, other_email_domain_block
           end