How Hard to Get Site Penalized on Purpose?

I have noticed that an unrelated domain name is now pointing to my site. The DNS is pointing the domain to my IP.

I'm not sure if it is a mistake, or if someone has done it on purpose.

It would seem to me this could make Google look at it as duplicate content, or a doorway site and potentially penalize my site! Is this possible?

Are there other ways someone could try to get my site penalized on purpose such as posting it on a notorious link farm?

One domain will not make a difference.

They are probably trying to get their domain indexed with your content.

You can block it with an htaccess redirect.

But what should I redirect it to? Anything? A 404 Error? My competitor? (the domain has nothing to do with my or my competitors business)