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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_handler(self, message_type, handler):
 """Manage callbacks for message handlers."""

        if message_type not in self._handlers:
            self._handlers[message_type] = []

        if handler not in self._handlers[message_type]:
            self._handlers[message_type].append(handler)