As @Slartybartfast said though Copy Paste is perhaps the wrong tool. Copy Paste is not the same as backup - there are too many considerations. If you want to backup and have a pre-defined strategy for existing files then use File History, or xcopy or robocopy or any of the hundreds of 3rd party backup softwares there are. If you don't know what you want to do with existing files on the backup you can think about it at the time.
Making copy/paste more complicated would not make anything better for anyone and if I was in charge of MS development I'd quite simply not do it.