One of the heroes of the Boston Marathon manhunt said it was a Florida cop’s “duty” to rush inside Marjory Stoneman Douglas High School last week and take on the gunman instead of hesitating outside.“You have to go in there and stop the killing. It’s your job and your duty,” said Dic Donohue, the MBTA cop who nearly died in the gunfight with the Tsarnaev brothers nearly five years ago.