cp and --backup error

[slash mais]

The command:
        cp -r -P --preserve=all --strip-trailing-slashes --backup=numbered
"/source-dir/source-file" "/source-dir/"
returns:
        cp: ‘/source-dir/source-file’ and ‘/source-dir/source-file’ are the 
same file

This result is wrong because '--backup' has been specified, and 'cp'
should do the backup and copy actions regardless in this case.