diff --git a/app/models/user/co_author_relation.rb b/app/models/user/co_author_relation.rb
index b4fb40ea4..f0d5f17c3 100644
--- a/app/models/user/co_author_relation.rb
+++ b/app/models/user/co_author_relation.rb
@@ -6,5 +6,6 @@ class CoAuthorRelation
 
   has_and_belongs_to_many :co_authors
 
-  validates :relation, presence: true
+  validates :relation, presence: true,
+            uniqueness: { case_sensitive: false }
 end