>
> 1 - select (gen_comuni_id,0) from rdb$database : value

This statement fails to call the gen_id function, and even if it did,
it adds zero to gen_comuni_id then returns it. Why would you expect
the value to change???

Try:
select gen_id(gen_comuni_id,1) from rdb$database

That should return the next value every time it is called.

Adam