It can be applied to pretty much anything. In most cases it's quite reasonable (there should be one and only one list of users of the system), but taken to the extreme (there should be only one list of messages your application may ever give to the user) can hinder the development a lot.
I think this appliesmostly to persistently stored data. Redundancy in your data model is dangerous because the copies of the data will eventually get out of sync, resulting in weird bugs. No matter how often and how many transformations you expose the data, make sure that it only exists once deep down in your model.
On one of my last jobs, I had to convince my teammates repeatedly that this is a good idea. I always showed them how this was simpler and more robust, yet a few weeks later, they were about to commit the same design errors.
