Star Trek: 10 Things You Didn’t Know About Paul Stamets
5. Mirror Man
Not bad breath, just pure bad, there was a lingering whiff, a grim presence left in the mirror at the end of Choose Your Pain, a reflection of things to come. Then a hint, later revealed, the man in the mirror was literally the mirror (universe) man — the Terran Empire alternate Stamets, whom we met properly (well, in the mycelial network anyway) in Vaulting Ambition.
If encountering his dastardly duplicate, who had been sending him messages from the network, was a new experience for Prime Stamets, Anthony Rapp had never acted with himself before either, as he revealed in an interview with TrekCore, calling the process "strange," but ultimately "very, very satisfying".
In order to mark a distinction between the two characters, Rapp focussed on how the "single-mindedness" and "manipulative nature" of the Mirror Universe might have shaped the personality of the Stamets there. One difference we do know of, and another first for Star Trek (just by saying the word), was that, according to Georgiou in The Red Angel, Mirror Stamets was "pansexual" and may have even had a fling with the former-Emperor.
Speaking of mirror men, the Star Trek: Discovery Annual 2018, a beta canon comic written by Kirsten Beyer and Mike Johnson, depicts the very first encounter between then Lieutenant Stamets and a certain Captain of the Discovery with a "condition" that means he likes to keep the lights down low.
I've also heard that if you say 'Stamets' five times in the mirror, the spirit of all the mushrooms you've ever eaten will appear to exact their fungal revenge.