Controlling duplicate key in Access DataBase - ms-access

Question

i have a problem with Access DB and VB6.
I have a program to make invoices, and the invoice number is a unique key.
The problem is when two people work simultaneously in a network introducing invoices.
I need to check that the invoice number entered by each of them does not exist to avoid duplicates.
Private Sub GuardarFactura()
If InvoiceNumberExist Then
MessageBox UserControl.hWnd, "Invoice number duplicate", "Control de errores", vbExclamation
Exit Sub
End If
On Error GoTo ErrorGuardar
Dim HayTrans As Boolean
AreaDeTrabajo.BeginTrans
HayTrans = True
Screen.MousePointer = vbHourglass
With recFrasEmi
.AddNew
!Numero = teInvoiceNumber
!Fecha = CDate(teFecha)
!TotalEuros = Format(CDbl(teTotal), FormatoImporte)
.Update
.Move 0, .LastModified
End With
AreaDeTrabajo.CommitTrans
HayTrans = False
DoEvents
Screen.MousePointer = vbDefault
Exit Sub
ErrorGuardar:
If HayTrans Then AreaDeTrabajo.Rollback
Screen.MousePointer = vbDefault
MessageBox UserControl.hWnd, Err.Number & ": " & Err.Description, "Control de errores", vbExclamation
End Sub
function InvoiceNumberExist
dim RS as recordset
Sql "Select * From Facturas Where Numero='" & teInvoiceNumber & "'"
Set RS = BD.OpenRecordset(Sql)
If Not (RS.EOF And RS.BOF) Then
InvoiceNumberExist=true
else
InvoiceNumberExist=false
End If
end function
InvoiceNumberExist don't work always if users work simultaneously,
I know i can use "if Err=3022 then ...." but is possible find the duplicate number before error??
Thanks

Answer 1

Thanks, then i think the only solution is use on error goto:
Private Sub GuardarFactura()
On Error GoTo ErrorGuardar
Dim HayTrans As Boolean
AreaDeTrabajo.BeginTrans
HayTrans = True
Screen.MousePointer = vbHourglass
With recFrasEmi
.AddNew
!Numero = teInvoiceNumber
!Fecha = CDate(teFecha)
!TotalEuros = Format(CDbl(teTotal), FormatoImporte)
.Update
.Move 0, .LastModified
End With
AreaDeTrabajo.CommitTrans
HayTrans = False
DoEvents
Screen.MousePointer = vbDefault
Exit Sub
ErrorGuardar:
If HayTrans Then AreaDeTrabajo.Rollback
Screen.MousePointer = vbDefault
If Err=3022 Then
MessageBox UserControl.hWnd, "Invoice number duplicate", "Control de errores", vbExclamation
Else
MessageBox UserControl.hWnd, Err.Number & ": " & Err.Description, "Control de errores", vbExclamation
End If
End Sub