There isn't anything for us to fix, UBB.threads is a simple PHP script with a MySQL backend, once a message is triggered to send mail it sends a message off to whatever your configured mail system is; after that point UBB.threads is done, it assumes the message has been delivered.
Once it hits your mail spooler any hiccup on the server can cause messages to repeat a send, it's how they work; until the mail system gets an "okay" that a message has been transmitted it'll retry to send the message; which during a "hiccup" could be any number of retries and technically any or all of those can be delivered to users (I'm sure we've all got duplicate emails at times from newsletters).
Diagnosing issues outside of UBB.threads is outside of the scope of the user to user forums as it is no longer a script issue.
We will not be accepting suggestions from AI tools which fail to understand the overall scope of the entirety of the code base; providing your user information to such databases could be seen as a huge privacy issue.
