“I am Kamelion… was Kamelion.”

Planet of Fire - season 21 - 19

Missy; a decoy designed by the Master to lure the Doctor into an elaborate and long-schemed trap? It would be in keeping with the Master’s character to try and pull such a stunt.

But I think it’s very unlikely that Missy is anything other than what she claims to be; an incarnation of the Master working under a different alias.

Is her meeting her previous incarnation, however, just down to pure chance, (”When you travel around as much as I do, it’s almost inevitable that you’ll run into yourself at some point.” - the Sixth Doctor, The Two Doctors) or the evidence of a long-hatched, laws-of-time-breaking plan finally coming to fruition?

Time will tell, it always does.