diff --git a/backend/db/migrate/20251011200300_add_discard_to_post_tags.rb b/backend/db/migrate/20251011200300_add_discard_to_post_tags.rb
index 11634ad..f825a37 100644
--- a/backend/db/migrate/20251011200300_add_discard_to_post_tags.rb
+++ b/backend/db/migrate/20251011200300_add_discard_to_post_tags.rb
@@ -1,5 +1,19 @@
 class AddDiscardToPostTags < ActiveRecord::Migration[8.0]
-  def change
+  def up
+    execute <<~SQL
+      DELETE
+        pt1
+      FROM
+        post_tags pt1
+        INNER JOIN
+          post_tags pt2
+          ON
+            pt1.post_id = pt2.post_id
+            AND pt1.tag_id = pt2.tag_id
+            AND pt1.id > pt2.id
+      ;
+    SQL
+
     add_column :post_tags, :discarded_at, :datetime
     add_index :post_tags, :discarded_at
 
@@ -15,4 +29,8 @@ class AddDiscardToPostTags < ActiveRecord::Migration[8.0]
     add_index :post_tags, [:post_id, :discarded_at]
     add_index :post_tags, [:tag_id, :discarded_at]
   end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration, '戻せません．'
+  end
 end