Sorry for being more curious than helpful, but why would you want one?
They are less common now than they were 10 years ago, and the new house I have just bought happens to have one, but not sure why?
I guess it is the practical benefit of enabling people to step in out of the rain while they're waiting for you to open the front door, as well as the fact that the double door will minimise heat loss.
I think they look crap, but if there is a good practical reason for having one, I won't complain.