That being said, whether data sharing is a good thing or not depends on the situation. For example, copying to a cache can expose more memory parallelism in an application. I feel like you are taking this opportunity to take a jab at FP here ("the purists detest mutation"). While FP is my preferred paradigm of development, I wasn't trying to push it on anyone--I was merely making a technical point.

Pure FP = don't mutate variables in the program. Of course the actual implementation can reuse memory blocks or else pure FPers would have to keep buying new memory!

In the face of these techniques (and others, e.g. re-structuring things to use zippers and being clever manually or whatever), I suspect the actual amount of redundant data might be quite a bit less than you might think.

For example, in programs with mutable state, it's not uncommon for modules to return deep copies of objects to avoid mutation-related bugs. Java's String objects are immutable precisely to avoid having to do this.