I used to work at a computer museum and we had an old computer from the 60s that was used in a plant and they came back and took it because they were upgrading their main computer and they wanted to keep redundancy I.E a backup for their backup.

In short, yes. They always have two machines running ready to swap in. I have heard some also run multiple live mainframes to check that the results agree, and discard and restart calculations if they don't.