In many cases trying to run the same amount on the same card within a certain timeframe can result in a message "Duplicate transaction declined" or the like.  The processors used by most Merchant providers have a mechanism in place to look for these "duplicate" transactions and stop them on the basis that they are quite often an indication of illicit activity on an account.  However, if you know you're going to be running identical transactions for whatever reason most Merchant providers will allow you to turn this feature off if you contact their customer support.