trigger checkEmailTrigger on Lead (before insert) {
// i used before insert trigger because if our condition is true then i have to stop inserting Lead 
//else this it will insert 

   final String errMsg = 'The email already exists on another Lead: '; // this is only for passing message using
  Set< String > emailSet = new Set< String >();
    for( Lead l : Trigger.new ){
        emailSet.add( l.Email );// in the set first i store all the email id which are comming in bulk to set 
    }
  Map< String, Id > duplicateLeadMap = new Map< String, Id >();
  // for bulkyfication of code need a strong logic so i created map where i put email id as key and lead id as      //value because a simple logic which always we used in college day for filtering  
  for( Lead L : [select Id, Email from Lead where Email = :emailSet] )
    duplicateLeadMap.put( L.Email, L.Id );
// after here i start checking the trigger.new values and if email id is in the map simple that mean it exists in 
// my lead object so i put my message in addError function which are used for showing error in apex 
  for( Lead L : Trigger.new ){
    Id duplicateContactId = duplicateLeadMap.get( l.Email );
    if( duplicateContactId != null )
      L.addError( errMsg + duplicateLeadMap );
  }
}