Same strain, different growth rates??

I had some Nirvana NL seeds left over from 5yrs ago and decided to grow them, but one of the plants is growing 2-3x the speed of the others, and they sprouted at about the same time. The cotyledons are starting to brown and dry up on the smaller plants, but on the one that's growing super-fast, it still looks green and fresh. Is it safe to assume that this is a male plant? Or is this an unstable strain?

if you plan on cloning then clone the one that grows better. one plant growing faster than the rest is normal

It's normal for plants to do that and to assume sex before they show will make waste for you, sometimes my bigger plants are the females.
Never kill until you see the balls.

Welcome to RIU
and like other guy said if you wanna clone something, always use the bigger better plant.