If I understand correctly, you want to use Grub to start a "copy" of the installed Debian OS for practice in such a way that, if you break it, it won't affect the existing install.
That would be asking Grub to do something it's not designed to do. It can't boot a copy of an installed OS into a sandbox of some sort. You can created a second pointer to the existing Debian install, but it will still boot the existing OS, not a sandboxed copy of some sort.
If you have enough RAMs, the best way to set up something for practice is to create a Virtual Machine using VirtualBox
or some other virtualizer. That way, if you break the VM, you can simply delete it and start over.
Alternatively, you make your computer dual-boot by installing a second OS, but a VM would be the best way to go.