If all of them suffer from the same issue (let's say memory corruption), you end up with 5 satellites that will fail on the same timeline.

When operating multiple devices to improve reliability, diversity is one point.

NASA doesn't know how to do anything cheaply. They have two space telescopes sitting in a warehouse since 2011 that were donated by the NRO. They're estimating $4 billion to launch and run just one of them, more than a decade after the donation. And you know it'll cost double the estimate and take an extra 5 years.

Why dont they just give others to do their work? Last I remember india sent a probe to Mars for less than amount it took for making the movie Mars featuring Matt Damon ?

For the politicians that fund NASA, spending money is the whole point. They call it "job creation".

agreed but what job creation is a project sitting dead for 5 years? are they giving people salary for not doing anything and then claiming too much costs or something else?