Why was I charged twice for my purchase or charged for a purchase that says it failed?

In both of these cases, when a purchase fails one of the banking fraud checks, your bank may place a hold on the funds for up to 5 business days. These funds will not be collected by Valve. Please contact your bank to see if they will release the held funds.

If you still see a pending charge after 5 business days, please contact Steam Support.

Source of the information and additional details can be found here.