I noticed something that looks strange to me, maybe because I'm not so familiar with Linux yet.

Supposing 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:

???

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:

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.

Thanks for any help.

You are mistaken.

How would the command know to exclude the BACKUP subdirectory?

You answered the question yourself.

When using the recursive option of cp, one must make the destination be in a different directory.

All that is saying is to copy EVERYTHING in this directory to NEW directory, which technically includes the directory you are trying to copy into. If anything, copy into an outside directory, or use something like What that does is copies everything except for files that start with . I use it for that except purpose, but maybe you can come up with something to suit your needs.

I'm not saying I'm not mistaken.
There obviously is a mistake and it is most probably on my side.

Actually, however, the command seems to know very well that the BACKUP subdirectory has an... ambiguous role in the operation. Otherwise, how could it show the message:
?

Anyway, if such a use of the cp command is erratic and can lead to unpredictable results, maybe the command should not accept such a command line or, at least, warn the user about the "danger".