It is not.
But, in order to successfully learn how to properly do doubles, you also have to learn and practice as there was no rebound.
In fact, both the videos you posted in the comments are suggestions about other ways to improve it.
The point is that the second stroke does use the rebound, but it cannot only rely on it. Practicing on a surface that offers little or no rebound allows you to improve the way you "reply" to the rebound (for both the upstroke and downstroke), improves the strength and endurance of muscles and tendons of your arms, wrists and hands, allowing you to only use the energy and movements required to properly play the second stroke, without wasting anything.
Also consider that, while the general technique is almost the same at any speed, it actually isn't, and deciding the amount of energy (and relying on rebound) it's up to you, your experience, your practice and the musical result you want to achieve.
Clearly, at higher speeds, you cannot control each second stroke as you would at slower speeds; you may try, but you have to be sure you're up to it, otherwise you could risk injuring yourself (tendinitis is a very bad thing).
At lower speeds you cannot obviously count on the rebound in any way, as there's no enough rebound "height" that could match the speed.
At "medium" speeds, choosing whether to use more rebound or more control can dramatically change the musical result and feeling, as the energy (and timing) that is put on the strokes is very different.
So, yes, you should also practice without rebound, but remember that it's only part of the practice.