One of Celestia's more ingenious ideas, she realized that the underpopulation problem in Equestia could be fixed by the simple addition of hundred of clones made of selected ponies.This led her to set up the Equestria Cloning Project.
This project has been going for some time, explaining how certain ponies do not seem to age, having been seen in flash backs as adult mares. After some consideration she turned this towards military applications as well, explaining why all of the Royal Guard look exactly the same.
Alternatively, some people have put forth that this phenomena could be down to lazy artists. These allegations are clearly seditionist lies.