Originally Posted by Google

Authorizations

Authorizations occur between our billing system and your bank's billing system to ensure that the card is valid and the necessary funds are available in your account before the order is charged. Because the order has been cancelled, the pending authorization will not result in a charge.
You may also notice a $1.00 transaction from Google, which is actually a pending authorization request between our billing system and the bank that issued your credit or debit card, so you won't end up paying the extra $1.00.
When you check your bank account or credit card statement, payment authorizations are often listed as "pending." These transactions might remain in your account between 1-14 business days, depending on your card-issuing bank. Please keep in mind they are authorization requests only, not actual charges.
Authorizations can remain in your account anywhere from 24 hours up to 14 business days, depending on the bank. You may contact your bank to confirm the charge for your purchase with Google or to check on the authorization status.

Duplicate Charges

While it may appear that you've been charged twice for an order, the pending item you noticed on your bank statement is actually an authorization. Authorizations are the initial communication between our billing system and the bank's billing system.
While you may see multiple pending listings in your account, only one charge will be deducted after the authorization is completed. When this final deduction occurs, the charge will be posted to your account as successful. Google will not bill an account until we receive an approved authorization from the bank.