Final update: I changed the duplicate checking system entirely and I believe I have fixed this once and for all.
You can now send an exactly identical tree mail (subject and/or body matching perfectly a previously sent mail) as long as the recipients are different.
So you can send me a mail and then send the exact same mail to someone else right away and they'll both be sent.
The only time the mail will be ignored and not sent is if you try to send the exact mail to the same person inside a 15 minute window.
