trigger validate on Contact(before update){
        SET<String> ids = new SET<String>();
            for(contact c : trigger.new){
                ids.add(c.AccountId);
            }
            List<AggregateResult> tempAggr = [SELECT AccountId acc FROM Contact WHERE In_Use__c= true AND AccountId IN : ids GROUP BY AccountId];

            Set<String> inUseset = new Set<String>();
            for(AggregateResult rs : tempAggr){
                inUseSet.add(String.valueOf(rs.get('acc')));
            }

            for(contact c : trigger.new){
                if(inUseSet.contains(c.AccountId)){

                    c.adderror('In Use already used by another Contact');
                    c.In_Use__c= false;
                }
                else{


                    inUseSet.add(c.AccountId);
                }
            }
        }