diff --git a/backend/app/models/tag.rb b/backend/app/models/tag.rb
index a1372c7..491a0e3 100644
--- a/backend/app/models/tag.rb
+++ b/backend/app/models/tag.rb
@@ -1,5 +1,5 @@
 class Tag < ApplicationRecord
-  class NicoTagNormalisationError < StandardError
+  class NicoTagNormalisationError < ArgumentError
     ;
   end
 
@@ -67,12 +67,14 @@ class Tag < ApplicationRecord
     @bot ||= find_or_create_by_tag_name!('bot操作', category: 'meta')
   end
 
-  def self.normalise_tags tag_names, with_tagme: true
-    tags = tag_names.map do |name|
-      raise NicoTagNormalisationError if name.start_with?('nico:')
+  def self.normalise_tags tag_names, with_tagme: true, deny_nico: true
+    if deny_nico && tag_names.any? { |n| n.start_with?('nico:') }
+      raise NicoTagNormalisationError 
+    end
 
+    tags = tag_names.map do |name|
       pf, cat = CATEGORY_PREFIXES.find { |p, _| name.start_with?(p) } || ['', nil]
-      name.delete_prefix!(pf)
+      name = name.delete_prefix(pf)
       find_or_create_by_tag_name!(name, category: (cat || 'general')).tap do |tag|
         if cat && tag.category != cat
           tag.update!(category: cat)
@@ -107,11 +109,13 @@ class Tag < ApplicationRecord
     (result + tags).uniq { |t| t.id }
   end
 
-  def self.find_or_create_by_tag_name!(name, category:)
+  def self.find_or_create_by_tag_name! name, category:
     tn = TagName.find_or_create_by!(name: name.to_s.strip)
     Tag.find_or_create_by!(tag_name_id: tn.id) do |t|
       t.category = category
     end
+  rescue ActiveRecord::RecordNotUnique
+    retry
   end
 
   private