The english equivalent of this sentence would be "she's a rare beauty," which is a compliment. I have to admit, to English ears saying "she's rarely pretty" implies that it's rare she looks pretty which would be seen as an insult. Languages all work differently though, and Norwegian and English are no exceptions.
Did Duolingo get an update recently that messed up numbers? In Spanish it's correcting 'third' as '3rd', and here it's correcting 'a' in "She is a uncommonly pretty woman" as "She is 1 uncommonly pretty woman." I realize 'a' is incorrect, but shouldn't it be 'an' or 'one' instead?
A screenshot of the issue: http://i.imgur.com/3RL5kFG.png