For most still photographers, the negative itself was the master archive. Before digital scanning became cheap and common, the usual approach was careful physical storage rather than routine backup.

If the originals were destroyed by fire, flood, fungus, or age, they were often simply lost. Many photographers kept prints, and sometimes extra print copies, but not duplicate negatives.

In professional motion-picture workflows, duplicate elements were more common: an original negative could be used to make an intermediate positive and then a duplicate negative for printing. That kind of duplication existed, but it was not typically how everyday still photographers archived their work.

So the short answer is: most people did not “back up” negatives in the modern digital sense. They stored them as safely as possible, because the original negatives had unique value and could not easily be replaced. Even very secure storage was never guaranteed to be safe.