Imported 5 new contacts and requested "Join new contacts to existing group(s)" in the Preview step. All 5 contact imported correctly and were placed in the new group. However the status message on the Summary page said:
Import to Groups : 5 contacts added to this new group. 5 contacts NOT added (already in this group)
The second part of this status is erroneous.
Peter Davis tested a similar scenario with 3 contact - where 1 contact was already in the DB. He got "2 contacts NOT added (already in this group)". So it looks like the erroneous 2nd count is counting newly inserted contacts who WERE added to the group and saying they're alread in the group.
You can use the attached CSV to test if needed (altho I think any simple CSV will work). I did not map the custom fields in this file. The mapping I used is currently saved on the demo as "DGG Import Test".