Topic title duplicated in header

Hello,

I just fixed this and submitted a pull request.

https://github.com/discourse/discourse/pull/4760

EDIT: To explain a little bit how I tracked the bug:

  1. I tried reproducing and found out that it actually reproduces by changing a topic’s title.
  2. I found out that there are two events that control the header and if the topic title is shown or not: header:show-topic and header:hide-topic.
  3. Searched where header:show-topic is triggered and found a match when a topic is reloaded (which happens when a topic is edited).
  4. I changed the code to trigger header:show-topic only if the the topic title is not visible (not the one from the header, but the proper one, #topic-title).
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