I have created a group, added multiple contacts (drag&drop), and the contacts delete from the group

I have created a couple groups, and spent quite some time adding contacts to these groups. While doing this a duplicate group heading creates itself? Also all the address I have added to the 2 groups just disappear. I added about 38 contacts to a group and then suddenly most of the contacts disappeared, deleted?? I have a new macbook pro OSX El Capitan