Disallow registering the same command several times

[jonahsnider]

import {Builtins, Cli} from 'clipanion';

const cli = new Cli({
	binaryLabel: `cli`,
	binaryName: `cli`,
	binaryVersion: `1.0.0`,
});

cli.register(Builtins.VersionCommand);
cli.register(Builtins.VersionCommand);
cli.register(Builtins.VersionCommand);
cli.register(Builtins.VersionCommand);
cli.register(Builtins.VersionCommand);

This should not be possible.