Prevent duplicate message on fetch/event race

[gnprice]

When the user navigates to a narrow so we fetch the messages there, and then a recent message in the narrow belatedly arrives as a MessageEvent, we currently end up showing the message duplicated in the message list. Instead, we should suppress the duplicate.

This seems to happen reasonably often in practice: @PIG208 saw it last week, and then I saw it today. We've also had code to handle it in zulip-mobile since forever.

Details in chat thread:
https://chat.zulip.org/#narrow/stream/48-mobile/topic/duplicate.20message.20from.20fetch.2Fevent.20race/near/1931798