Occasionally a bank will block a transaction because they consider it to be suspicious. This is done to protect you. However, it can be annoying if a legitimate transaction is blocked.
When this happens, the blocked transaction may appear in your bank statements for a few days, even though the transaction has not gone through.
Also, when this happens, sometimes people get confused and try the purchase twice. Then, if the customer contacts their bank and the bank unblocks the transactions, both attempts to pay may be processed, or appear to be processed.
If this happens to you, and you appear to have been charged twice, please go to your Leanpub library, select one of the purchases, and request a refund.
The refund will be processed automatically, but it may take a few days before this is shown correctly in your bank statements.
If you are seeing "This transaction cannot be processed" please read this article for some help: http://help.leanpub.com/reader-help/im-trying-to-buy-a-book-and-i-see-this-transaction-cannot-be-processed.