fix duplicated comment validations
fix duplicated comment validations

We ensure there is a `last_comment` before going further.
Commit bbe16ec breaks 64 specs and seed data.

vveliev/teambox,irfanah/teambox,rahouldatta/teambox,rahouldatta/teambox,teambox/teambox,nfredrik/teambox,nfredrik/teambox,nfredrik/teambox,vveliev/teambox,irfanah/teambox,teambox/teambox,rahouldatta/teambox,irfanah/teambox,teambox/teambox,vveliev/teambox,vveliev/teambox,irfanah/teambox,nfredrik/teambox