Avoid re-registering a message type already registered (#6354)

This change avoid re-registering a structured message already registered
to the team by a previous agent also included in the team.

This issue occurs when agents share Pydantic models as output format

## Related issue number

Closes #6353

---------

Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>

Author: jorge-wonolo

python/packages/autogen-agentchat/src/autogen_agentchat/teams/_group_chat/_base_group_chat.py

@@ -73,7 +73,8 @@ def __init__(
         for agent in participants:
             for message_type in agent.produced_message_types:
                 try:
-                    if issubclass(message_type, StructuredMessage):
+                    is_registered = self._message_factory.is_registered(message_type)  # type: ignore[reportUnknownArgumentType]
+                    if issubclass(message_type, StructuredMessage) and not is_registered:
                         self._message_factory.register(message_type)  # type: ignore[reportUnknownArgumentType]
                 except TypeError:
                     # Not a class or not a valid subclassable type (skip)