insert on duplicate with a where clause on a field not part of the unique key

[Ravi Malghan]

Looking for some assistance with a insert statement on duplicate key. I have this table

CREATE TABLE IF NOT EXISTS `alarm` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `event_id` varchar(30) NOT NULL,
  `hostname` varchar(32) NOT NULL,
  `status` tinyint(1) DEFAULT '0',
  `clearedtime` datetime default NULL,
  `count` tinyint(1) DEFAULT '0', 
  PRIMARY KEY (`id`),
  UNIQUE KEY `no_duplicate` (`event_id`)
)

I want to insert a new row if a row with same event_id exists and clearedtime is not null. clearedtime cannot be part of the unique key. If the row exists with a clearedtime null, I want to increment the count by 1. Can't seem to figure out how to include claredtime != null

insert into alarm (event_id, hostname, status,clearedtime, count) values ('event1', 'hosta', 5, null, 1) on duplicate key update count=count+1