A friend of mine gave me this explanation:
The simplest solution is that all of this appears to be a reference to something mentioned in the novel, “Interference.” In that novel, we learn that while preparing for the Time War the Time Lord undertook a Hatching Project where they made clones of important Time Lords to serve as warriors in the War. It’s stated that the Master and the Rani were cloned but implied that there could have been many others. These clones appear to have had the original’s memories (otherwise they wouldn’t be much help).
I’d guess that the clones were created using some temporal duplication of biodata rather then traditional genetic growth.
For what it’s worth, the Faction Paradox stories imply this happened about 137 years before the Time War began – when the Time Lords first became aware that the War was coming.
Alternately, they might have removed the original Master from the singularity of the Doctor’s TARDIS in a manner similar to how the Doctor escaped a black hole in “The Three Doctors” on using temporal bonding (probably to the Doctor) like Omega used in “Arc of Infinity.”
Either solutions are possible but the use of the word ‘Resurrected’ combined with the explicit mention of the Time Lords making a Master clone to fight in the War in “Interference”, has me leaning on that solution and the most likely.