Orders comes twice

I got a problem with the shop on this site: hippus.ch/ (joomla2.5):
Sometimes orders come twice, and i don't understand why; i don't think that customers have made ​​a mistake, because the situation has occurred several times.
I found a topic that suggests to disable the joomla cache, but i don't have cache active.
Any suggestion?
Thanks

Hello,
Can you download the last Hikashop version through our website, install it and test it again ?
Thank you.