What was found
IN CRUISE FLIGHT, PAX ALERTED CREW THAT UPPER PORTION OF CABIN DOOR APPEARED TO BE UNSECURE. SIC WENT TO SECURE DOOR, PIC INITIATED AN IMMEDIATE DECELERATION & DESCENT INADVERTENTLY INDUCING A MAIN ROTOR OVER SPEED. DOOR WAS SECURED & FLIGHT CONTINUED TO DESTINATION. UPON SHUTDOWN OVER SPEED WAS DISPLAYED ON HUMS. THIS WAS THE 3RD OR 4TH INCIDENT OF THE DOOR OPENING ON THIS ACFT.