Duplicate is in the Standard Suite, as well as delete, exists, make, move which apply to files.
Delete, move are also in Disk-Folder-File Suite but exists and make aren't.
So, with two functions matching your rule and two which don't it was not clear if duplicate belongs to one set or to the other one.
Your understanding is that it's normal that duplicate doesn't apply to files.
Mine was different. Maybe you are right, maybe you aren't but don't worry, I will live with this behavior.