A second pump is one option, another option is some kind of flow rate, temperature or pump RPM monitoring system.
You can use several programs to shut the computer down if the CPU / GPU / whatever temperature exceeds a set point. This will shut the computer down in case of pump failure.
The pump RPM wire (if it has one) can be plugged directly into a motherboard fan header, you can then use software, such as speed fan, to shut the computer down if that falls below a certain level.
You can get flow sensors which can be plugged into fan headers on the motherboard, although temperature sensors need to go through a controller like the Aquero, BigNG or Heatmaster. These can also be used to shut down the computer if the flow rate or water temperature go outside set limits.
If you do want to get another pump it does not have to be a D5; so long as the overall loop flow rate does not exceed the lowest rated flow of either pump you are fine. Martin did his dual pump test with a D5 and a DDC with no ill effects, in theory you could easily run a D5 and something like a Jingway 1200 in the same loop.