Okay, so technically a fractal is any shape of non-integer dimension, but assuming you're going for the more well-known definition of infinitely self-containing...
Whenever you destroy a Fractal Door that still has copies left, instead of simply reforming into the same door it was before, it reforms into a Combo Door that contains multiple of the door it was before, with that amount being whatever its spend cost was originally; its spend color remains the same as it was before. If it was a 2 Purple Door, it reforms into a Combo Door with two locks, both of which are 2 Purple, and its spend color is also purple. Destroy that door, and it reforms into a Combo Door with two copies of that door, for a total of four 2 Purple locks (it reforms with two copies, not four, because its spend cost was 2 originally). Destroy a 3 White Door that's Fractal, and it becomes a Combo Door containing three 3 White locks, and destroying it again will increase that to nine 3 White locks. Destroy a Fractal Combo Door with a 4 Red lock and a -1 Blue lock, the door now has three of each of those locks. Destroy a Fractal Combo Door with a 5 Black lock and a -5 Orange lock, and the amount of locks is multiplied by zero so now it's a Lockless Door. Destroy a -2 Green Door, and the amount of locks is multiplied by -2, so now it's a Combo Door with two positive 2 Green locks. I'm not sure how the count of locks turns out once Blast Doors get involved...