Just to be contrary, I think that there is no point to learning how to double tongue until the student has "hit the wall" tempo-wise with his single tongue. I was taught double tonguing WAY too early and developed a lazy, slow tongue. The underlying reason that my single tongue was slow was because it was flawed: I was anchor tonguing and didn't realize it until I was a sophomore music major in college. My nick name should have been "Chip". Because the single tonguing was poor, the double tonguing was simply fast and poor. Nothing got straightened out until I corrected my fundamental single tonguing technique as which point EVERYTHING fell into place from an articulation standpoint.