diff --git a/app/models/draft.rb b/app/models/draft.rb
index 2bbc3bcaf6..4f7a03711f 100644
--- a/app/models/draft.rb
+++ b/app/models/draft.rb
@@ -282,5 +282,5 @@ end
 #
 # Indexes
 #
-#  index_drafts_on_user_id_and_draft_key  (user_id,draft_key)
+#  index_drafts_on_user_id_and_draft_key  (user_id,draft_key) UNIQUE
 #
diff --git a/db/migrate/20191108000414_add_unique_index_to_drafts.rb b/db/migrate/20191108000414_add_unique_index_to_drafts.rb
new file mode 100644
index 0000000000..f50d1b7c49
--- /dev/null
+++ b/db/migrate/20191108000414_add_unique_index_to_drafts.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+class AddUniqueIndexToDrafts < ActiveRecord::Migration[6.0]
+  def up
+
+    execute <<~SQL
+      DELETE FROM drafts d1
+      USING (
+        SELECT MAX(id) as id, draft_key, user_id
+        FROM drafts
+        GROUP BY draft_key, user_id
+        HAVING COUNT(*) > 1
+      ) d2
+      WHERE
+        d1.draft_key = d2.draft_key AND
+        d1.user_id = d2.user_id AND
+        d1.id <> d2.id
+    SQL
+
+    remove_index :drafts, [:user_id, :draft_key]
+    add_index :drafts, [:user_id, :draft_key], unique: true
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end