index should have always been unique

db/migrate/20170222173036_make_user_auth_token_index_unique.rb

@@ -0,0 +1,12 @@
+class MakeUserAuthTokenIndexUnique < ActiveRecord::Migration
+  def up
+    remove_index :user_auth_tokens, [:auth_token]
+    remove_index :user_auth_tokens, [:prev_auth_token]
+    add_index :user_auth_tokens, [:auth_token], unique: true
+    add_index :user_auth_tokens, [:prev_auth_token], unique: true
+  end
+
+  def down
+    # no op, this should always have been unique
+  end
+end