This is bad, as it have unneeded error in status-vector :

DDL: CREATE TABLE T1 (ID INT)
DDL: CREATE UNIQUE INDEX T1_UQ ON T1 (ID)
DDL: COMMIT


tx1: SET TRANSACTION READ COMMITTED RECORD_VERSION NO WAIT
tx1: INSERT INTO T1 VALUES (1)
tx2: SET TRANSACTION READ COMMITTED RECORD_VERSION NO WAIT
tx2: INSERT INTO T1 VALUES (1)
        lock conflict on no wait transaction
        attempt to store duplicate value (visible to active transactions) in unique index "T1_UQ"

More, if someone tried to catch isc_no_dup and handle it in some way he well be in trouble.