I got a duplicate when creating a list via Twitter.com (I’ve also had this when creating them from SproutSocial).
This recent time, I added a couple of people to one of the lists - the two lists were visible, one listed as having a couple of members, one listed as empty.
However, when I clicked on either list name (even the “empty” one), I still saw the same two members. I tried to delete both lists to correct it, however I could only delete the one listed as having members - the other won’t delete. I now have this orphan list there that I can’t add people to, and can’t delete.