Affected rows count by rule as condtition

Hi,
is there any way how to count affected rows by on update rule and use it
as part of condtions.

Example:

CREATE OR REPLACE RULE "_UPDATE" AS ON UPDATE TO "users" DO INSTEAD (
UPDATE "s_users" SET
id = new.id,
login = new.login,
WHERE id IN (SELECT id FROM "s_users" ) AND 2 > (SELECT count(new.id)) ;

Error: agregate functions not allowed in WHERE statement

It need to simulate unique constraint on field s_users.new_id, so it
should deny to update multiple rows with same value.

Any suggestions are welcome.