From what I know, space scientists always work on two prototypes just in case something goes wrong. Why wasn't this the case this time? and are there plans for a repeat of this mission soon?
Schiaparelli was a demonstration mission, not a science mission, and as far as I know there are no plans to repeat it.
There are however plans for a joint ESA-Russia Mars lander, currently planned to launch in 2020. That one is a science mission, larger than TDM with a different architecture. Interestingly, that 2020 lander plans to inherit the guidance system from TDM, the one thing that was known to fail on TDM. Fortunately it was also the thing that they received a lot of telemetry on before it crashed, so it is very likely that they can diagnose and remedy what happened.
Unfortunately the TDM did not complete the later phases that that guidance system was reponsible for, in particular the touchy operation of propulsively bringing the vehicle to near-zero vertical and horizontal velocity at near-zero altitude using the RADAR and IMU data. Since that phase was not validated by TDM, I'd expect that they're going to be planning a lot more testing on Earth of that phase before relying on it for their 2020 lander. (Or maybe it will become their 2022 lander, if this or a combination of things results in a delay.)
Space scientists always work on two prototypes just in case something goes wrong"
Well, this assumption is partially wrong.
First, it is already very expensive to create one mission, let alone make a backup for it. Second, you often only get one chance to get it right:
- You might not have the funding for another launch, even if you have the hardware.
The launch windows to other planets are usually very small (in the order of months); so you don't have enough time to prepare a second launch (provided you can figure out the issue and fix it before).
- Moreover, by the time you are ready for the second launch, science will have advanced, making your probe already partially obsolete.
There are many examples of such failures.
The latest ones being:
- Schiaparelli 03/14/2016 ✝ 19/10/2016
- Hitomi 17/02/2015 ✝ 27/03/2016
- Fobos Grunt 09/11/2011 ✝ 15/01/2012
While Schiaparelli was mostly a success despite the crash landing, Hitomi failed before it started operational life and no replacement is scheduled (some science will be achieved tho).
Pray that nothing will happen to the > $9 000 000 000 James Webb: We don't have a James Webb Junior.