Fix mattermost import handling of duplicate huddle objects

[timabbott]

See https://chat.zulip.org/#narrow/stream/31-production-help/topic/Migration.20from.20Mattermost/near/1288112 for background. But basically we believe that it's possible for the Mattermost=>Zulip data conversion tool to generate two copies of the same huddle, whereas these are intended to be a singleton for each set of user IDs.

@ganpa3 FYI -- this is likely a relatively localized fix.

[zulipbot]

Hello @zulip/server-misc members, this issue was labeled with the "area: export/import" label, so you may want to check it out!