Developers do not really have to be siloed to experience code duplication. When the team size grows past a certain point such that each person is not aware of what every one else is working on, code duplication is quite inevitable. This is the case even if everyone writes functional style code. In fact this just happened last month at work: I wrote a new functional and pure helper function and placed it at the beginning of the file; a week later a colleague told me a similar helper function with substantially the same functionality with a different signature had been written and placed near the end of the same file.