[TYPO3-commerce] Duplicated orders in database!!!

[Bert Hiddink [BENDOO e-work solutions]]

Hello,

I am running a Commerce-shop with TYPO3 4.3.1, added "com_ordernumber" 
for "friendly" ordernumbers and an extra extension we wrote to apply 
some hooks. We offer 3 paymentmethods: debit, iDeal and invoice. We use 
the Commerce SVN-version of December 2009.

During testing-fase, everything worked fine but as soon as we put it 
into production, we noticed that around 1-2% of the orders get 
duplicated in the database, that is, the customer makes 1 order but this 
gets registrated in the database as 2, 3 or 4 different orders, each 
with a unique ordernumber. Only the latest order (highest ordernumber) 
is send to the customer per email.

As you will understand, this makes ordermanagement quite messy: we have 
to look for duplicates by hand and then remove them.

What could be wrong here? I really do not understand why this happens? 
How to debug?

Some extra comments:

*) The shop was heavily visited just after launch (3-4 orders per 
minute), more then 1000 orders during the first day. Could this be a reason?

*) On the page were the basket is installed, we had to set it to "No 
cache" due to some strange caching behaviour?

*) How heavily Commerce depends of cookies and/or js being enabled?

As you notice, the above are just some guesses since I do not see a 
clear pattern between cause and consequence.

Any idea on this would be highly appreciated.

Thanks and regards,

Bert Hiddink
BENDOO