First time i use:
myCashBookEntry = _session.CashBookEntry.CreateFinanceVoucher(cashBook, myAccount, myContraAccount);
Second time i use
ICashBookEntryData myCashBookEntryData = _session.CashBookEntryData.GetData(myCashBookEntry);
//some updates to myCashBookEntryData and then i do this
_session.CashBookEntry.CreateFromData(myCashBookEntryData);
Then i get the error: "The entity already exists"

How should I do?
I want to create several rows with the same verification number.
If i use the first line for every row i t works but the i get different verification number for each row and i want the same number for every row.