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.
```