How to write trigger to avoid duplicate email when bulkfying the trigger

my trigger is:

trigger AvoidDuplicate on contact (before insert,before update)
{
   if(Trigger.isInsert||Trigger.isUpdate)
      for(contact a:trigger.new)
     {
         integer count=[select count from contact where email=:a.email];
         if(count>0)
          {
                 a.Email.adderror('This email already exists');
          }
     }
}

this is working fine when inserting single records

but it shows error when working with dataloader...... Please help to to over come this error


I would appriciate for any kind of replay...............