Missions to other worlds were from the beginning at the cutting edge of technology...We must expect a steady background of mission failures when we explore other worlds...Knowing about irreducible risks, why do we these days fly only one spaccraft per mission?...

...Consider the 1971 Mariner 8/Mariner 9 dual launch mission to Mars...Mariner 8 fell into the ocean. Mariner 9 flew on to Mars and became the first spacecraft in human history to orbit another planet. It discovered the volcanos, the laminated terrain in the polar caps, the ancient river valleys...If we had launched only Mariner 8 the endeavour would have been an unmitigated failure. With a dual launch it became a brilliant and historic success.

(Carl Sagan Pale Blue Dot.)

A lesson that clearly has not been learnt as yet.