Most software fails differently: in 95% cases it works 100% of the time, in 5% cases it always fails. You can't use two identical pieces of software for redunda

Most software fails differently: in 95% cases it works 100% of the time, in 5% cases it always fails. You can't use two identical pieces of software for redundancy. So this kind of redundancy only makes sense in rare cases: 1. There are two different pieces of software that do the same thing. 2. They are interchangeable enough, you don't need a fat adapter for each one. 3. The fault is not in one of their shared dependencies. 1 and 3 are rare because we try to reuse as much software as we can. That said, this does happen a lot with web standards: if a standard exists as an upgrade over a previous standard, being able to fall back to the old standard is a common requirement.