[Pidgin] #15458: Avoid duplicate MUC history messages

[Pidgin]

#15458: Avoid duplicate MUC history messages
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 Reporter:  Kha          |      Owner:
     Type:  enhancement  |     Status:  new
Milestone:               |  Component:  pidgin (gtk)
  Version:               |   Keywords:  muc history backlog
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 This is a port of #a14219. However, whereas the Adium path mentions it
 doesn't work across sessions yet, this one seems to automatically do so
 because {{{PurpleChat.components}}} appears to be persistent.

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Ticket URL: <https://developer.pidgin.im/ticket/15458>
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