var add = ['email1@gmail.com', 'email2@gmail.com', 'email1@gmail.com'];
var main_messages = []
var from
function findMessages(messageList) {
  return messageList = from;
}

add.map(function(map){
  from = map
  if(main_messages.find(findMessages) === undefined){
    main_messages.push(map)
  }
});

console.log(main_messages)

['email1@gmail.com', 'email2@gmail.com']

['email1@gmail.com']