General :: "cp" Command - Called BACKUP And Is Initially Empty

I'm in a directory containing various files and subdirectories, one of which is called BACKUP and is initially empty, what exactly happens if I launch the following command line:

Code: cp -R . BACKUP???

If I'm not mistaken, all the content of the current directory should be copied into the BACKUP directory, apart from the directory BACKUP itself. Actually I get a message saying something like:

Code: cannot copy a directory onto itself BUT when I look into the BACKUP directory I find another directory called BACKUP which I did not expect to find. Moreover, if I launch the same command again I get the same message (repeated two or more times) and then I find that inside the "second" BACKUP directory there is a third one, with the same name again. In other words, every time I launch the command I get a deeper tree of "BACKUP" directories, as if the command was messing everything up. I know that most probably I'm the one who's messing things up and I also know the "problem" can be easily bypassed by using as a destination for the copy a directory located elsewhere, but I'd like anyway to understand the reason for such a behaviour.