Emit WARNING for the cmdline options that exist multiple times

Don't emit ERROR here. Even GitHub Actions machines have repeated options
in /proc/cmdline.

Also see the comment in cmdline parsing for x86:
https://elixir.bootlin.com/linux/v5.16.7/source/arch/x86/boot/cmdline.c#L21
```
In accordance with standard Linux practice, if this option is repeated,
this returns the last instance on the command line.
```

kconfig_hardened_check/__init__.py

@@ -194,7 +194,7 @@ def parse_cmdline_file(parsed_options, fname):
                 name = opt
                 value = '' # '' is not None
             if name in parsed_options:
-                sys.exit(f'[!] ERROR: cmdline option "{name}" exists multiple times')
+                print(f'[!] WARNING: cmdline option "{name}" exists multiple times')
             value = normalize_cmdline_options(name, value)
             parsed_options[name] = value