All messages have an email_message_id provided by its sending email client that is included in its header. This email_message_id will be the same for every receiver of this message.
If you want to determine if messages are different, a great way would be to check the message_id. This is a globally unique ID that we assign to every message of an end user. If two end users have the same message from a sender, although those messages will have the same email_message_id, they will each have different message_id's.