A B757-200 PUSHING BACK FROM THE GATE AT MIA INCURRED MINOR DAMAGE WHEN THE GATE AGENT OPENED THE ACFT DOOR ATTEMPTING TO BOARD LAST MIN PAX.

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WE WERE DELAYED DEPARTING DUE TO LATE UM'S HAVING BEEN DELAYED AT CUSTOMS/SECURITY. I WAS SURPRISED THAT WE WERE SUBSEQUENTLY TOLD BY THE GATE AGENT WE WERE GOING TO DEPART WITHOUT THE LATE PAX. THE 1L DOOR WAS CLOSED BY THE AGENT AND THUS CLRED ON OUR WAY. WE THEN WENT THROUGH THE STANDARD CLRNC TO PUSH FROM THE GATE. THE BRAKES WERE RELEASED AND THE TUG BEGAN TO PUSH THE ACFT AWAY FROM THE GATE. THE ACFT HAD BARELY BEGUN TO MOVE BACKWARD WHEN MY FO TOLD ME THEY WERE GOING TO BOARD SOME ADDITIONAL PAX AND TO STOP THE PUSH. I IMMEDIATELY INSTRUCTED THE PUSHBACK CREW TO STOP THE PUSH. SOMEWHERE DURING THE PROCESS OF MY TELLING THE PUSHBACK CREW THAT THEY WERE CLRED TO PUSH AND THE INITIAL ROLLBACK OF THE ACFT THE FO WAS NOTIFIED BY THE #1 FLT ATTENDANT TO STOP THE PUSHBACK. BY THE TIME THE FO HAD RECEIVED INSTRUCTIONS FROM THE CABIN TO STOP THE PUSH FOR ADDITIONAL BOARDING OF PAX ALONG WITH HIS RELAYING THAT SAME INFO TO ME AND MY HAVING THEN RELAYED THIS SAME INFO TO THE PUSHBACK CREW; ALONG WITH THE REACTION TIME OF THE PERSON PUSHING US; THE ACFT'S LEADING EDGE OF THE L1 DOOR WHICH HAD ALREADY BEEN OPENED BY THE AGENT JUST PRIOR TO THE ACFT MOVING BACKWARD; HAD STRUCK THE JETBRIDGE; PARTICULARLY AT THE JETBRIDGE LEVELING DEVICE PORTION. AT NO TIME DURING THIS SEQUENCE DID I SEE OR HEAR FROM THE GATE AGENT WITH REGARD TO REOPENING THE 1L DOOR. IT WAS A COMPLETE SURPRISE FOR ME TO LEARN THE 1L ENTRY DOOR HAD REOPENED AFTER WE HAD ALREADY RECEIVED CLRNC TO PUSH. AFTER A THOROUGH INSPECTION BY MAINT OF THE ACFT AND JETBRIDGE IT APPEARED THAT THERE WAS NO APPARENT DAMAGE TO EITHER. NO APPARENT INJURIES OCCURRED AS WELL; THOUGH THE AGENT WAS AT HIGH RISK OF INCURRING ONE. AFTER ABOUT A 1 HR DELAY FOR ACFT INSPECTION; WE WERE THEN ABLE TO DEPART FROM THE GATE FOR THE REMAINDER OF OUR SEQUENCE.