> when a SELECT statement uses an alias named "field2" as
> first column, firebird reports an error:
> "A component named Field2 already exists"

Sounds like a development environment or utility error.
I just tried:

SELECT EMP_NO as field2
, LAST_NAME as field3
FROM EMPLOYEE

with FB v1.5.2 under WinXP with no error (using IB_SQL).

(If you did not get an extended error code (one of those
3355nnnnn numbers) it is pretty certain it did not come
from Firebird.)

--
Geoff Worboys
Telesis Computing