-->Banking application for sure will have option to reject the duplicate payments in case the files are not archived from the gateway server which in turn will be the response status payment messages which will be the separate interface in your case.
-->banks will not allow to acces their network directly so you will need to have a gateway server which will have firewall open between gateway and bank network this is all because of security
-->bank suppose to share the key to encrypt the data which you can use it in PI
-->If file is generating in SAP ECC AL11 then it supposed to have the specific file name please check with the functional team or refer the functional doc once again for the file name.
Good Luck
cheers
Pawan