Certainly seems like the wrong thing to do. A good test is that if you think it may be dodgy it probably is. I certainly wouldn't recommend it as a tactic. There are potentially multiple issues with this....duplicate content as you mentioned but also dilution of real links. Good quality legitimate links could link to the Ghost site and therefore not count for the real site.

I have seen issues where it is a legitimate attempt to have a .com and .co.uk on the same shop and ended up with both versions online due to incompetent development but I didn't have to deal with cleaning it up.

Un-picking that could be messy. A good example of quick fix SEO for a fast buck I suspect.