Magento checkout button available after click

I've got a problem when it comes to the last step at checkout in Magento.

When the customer pushes the "Place Order" button, it disappears, just as expected, but after a few seconds it become available again for a while until the final redirection to the payment service is made.

This is not a huge problem, but customers could click the "Place Order" twice or more which, I guess, can cause troubles.

Anyone knows how to fix this?

See this short video for a demonstration: 开发者_如何学运维Checkout button disappear/reappear

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See: http://www.magentocommerce.com/boards/viewreply/269181/