How can I reply as linked topic without having the linked topic included a second time in the main text window? I basically want to leave out "Continuing the discussion from ..." but still have the link appear on the right side.
Basically it should look like this:
Discourse links threads if they are mentioned from/to another post. Without the link, it just isn’t listed on the right.
I think it’s good Discourse does this, since we know that the topic is based on an existing topic, but goes off in a different direction to the original while keeping topics nicely linked together. 
