OFX interpretation query

[A.J. Bonnema]

Hi all,

Sorry for the long mail. I need to explain before I can ask the question.

I created a program to convert downloaded Rabobank csv file to OFX 
transactions. It works admirably.

The question is transaction identification. Each OFX transaction has an 
XML element called FITID, probably to identify the transaction. I 
assumed Gnucash uses it, but I think I was wrong.

I create a FITID by constructing the FITID: date + amount + C/D. So, if 
I would ever have the same amount from the same source on the same date, 
Gnucash should identify them as duplicate. Or so I thought.

Now, I usually import data from the 1st of January until year to date. 
That way I get complete transactions for the current year. Gnucash 
correctly ignores the transactions that were already imported, so I 
assumed it was using the FITID from OFX.

Last september I had two identical transactions as far as date and 
amount are concerned with identical FITID, so I expected Gnucash to 
ignore one of the two transactions, but it did not!

It turns out that Gnucash processes both transactions as different posts 
correctly and in future imports ignores both transactions correctly.

How is that possible? Can anyone explain how this mechanism works in 
Gnucash?

Kind regards, Guus Bonnema.