Prevent user from creating duplicate labels.

This isn't enforced at the data layer just in the UI's that
allow editting of the group name.

Bug 28718604
Change-Id: I969b0298bd895fdbdd7f8609da05f2b801707c0a
(cherry picked from commit 8d62cd2c5c855fd5483c030c1d59e50830ce4dfc)
11 files changed