#174 完了

backend/app/models/tag_implication.rb

@@ -2,7 +2,7 @@ class TagImplication < ApplicationRecord
   belongs_to :tag, class_name: 'Tag'
   belongs_to :parent_tag, class_name: 'Tag'
 
-  validates :tag_id, presence: true
+  validates :tag_id, presence: true, uniqueness: { scope: :parent_tag_id }
   validates :parent_tag_id, presence: true
 
   validate :parent_tag_mustnt_be_itself