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.