Duplicate a record don't increment a calculated file number

Hi,

I've a "Trigger on Create" with a formula to increment a number in a numeric field (I want to avoid the use of table ID). The trigger works fine if i click in a new record button, but if i duplicate a record (sometimes very usefull in my specific caseI it doesn't. Someone has discovered something similar. How can i avoid that without disable the duplicate button? The formula I've is very simple but i think it's ok ... or not?

let a := max((select Incentives).IID);
IID := a + 1