Btrieve Error 5 on Create View

Hi.
I have a select command that work fine.
No duplicate values.
If i want to create a view i receive Btrieve error 5. Duplicate Key
Error, SQL State 1000.
I dont understand.
Anyone can help me ?
Btrieve 8.59.189.00


Thanks in advance.
Alessandro Zambonin

Create View RIWTE_vwRigFatt (IdFattura, Iva, EsenteIva, Conto, Costo )
as

SELECT FR.IdFattura, FR.Iva, FR.EsenteIva,
if(CC.ContoEsatto is null, PA.ContoGenerico, CC.ContoEsatto) as
Conto ,
Sum(FR.Quantita * FR.Prezzo) as Costo
>From RIWTE_FattRec FR left outer join RIWTE_CorrConti CC
ON FR.Codice = CC.Codice
Inner Join RIWTE_Parametri PA on PA.Id = 1
group by FR.IdFattura, FR.Iva, FR.EsenteIva, if(CC.ContoEsatto is
null, PA.ContoGenerico, CC.ContoEsatto)