If you are using the Cloud Feeds feature in Core, you must have downloaded your credit card transactions in it. In case you lose your credit card and get it reissued, you need to reconnect Core to that replaced credit card because it is considered as a new account. When doing so, it downloads the previous transactions as well and displays them as new entries.
In this case, you should not re-add those transactions to your Core accounts. You can simply exclude the transactions in the Cloud Feeds screen. Please check the Core Help > Accounting > Cloud Feeds > Exclude Transactions for more on how to do that.