PROBABLE CAUSES:

Too many "Previous page" and "Next page" or a Page refresh may have corrupted data of your order. It is strongly suggested that you refrain from using these commands.

Navigating under numerous tabs could erroneously duplicate data; opening new tabs while placing an order is not recommended.

It may also be because "cookies" have been disabled or you took to much time filling the order form: the cookie that identifies your data is not permanent and is deleted about 25 minutes after it sees no activity.