Problem deleting record on the same table that the trigger fired

[Chan Nan]

Hi,

My scenario is to delete old duplicate records before insert the new ones (in another word reuse the name here). However, I got an error when running it.

Here's the info (please bear with me if I don't explain it clearly)

table mytable:
user_id, proj_id, item_id, data1, data2, data3
(note - user_id, proj_id, and item_id can have duplicate values
item_id shoud be reusable as a group)
ie.
user_id proj_id item_id data1 data2 data3
1 100 1234 hello Paris Food
1 100 1234 bye Chicago fruit
1 100 1234 hey New York beef
2 100 2222 A B C
2 100 2222 B B C

I'd like to write a trigger so that when three new records come in
like:

1 100 1234 bonjour Paris Food
1 100 1234 hola Chicago fruit
1 100 1234 whatup New York beef

the old ones should be replaced and I should have:
1 100 1234 bonjour Paris Food
1 100 1234 hola Chicago fruit
1 100 1234 whatup New York beef
2 100 2222 A B C
2 100 2222 B B C

Here's the trigger:
....
delete from mytable where user_id = new.user_id and proj_id=new.proj_id and item_id=new.item_id (delete old data if duplicate before insert new ones)
....
and the error:
ERROR 1442 (HY000): Can't update table 'mytable' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.

Does the error mean "I can't modify the table while the trigger is fired?".
I'd like to know if this is the way trigger was designed or I missed something. I know this can be handled fairly easy in the application side but it would be nice to do it within the trigger.

Thanks a lot in advanced.