Duplicates with Ning to Buddy Press Importer

I have a question about the Ning to BuddyPress User Importer that I would like to know before I use it for the first time.

If I have some users already on my BuddyPress installation with the same username as one that comes in from the importer, what happens?

I would prefer that the plugin notices the Ning user is a duplicate and ignores it.