How can I duplicate a topic?

You can do it in two ways:

1) Select a topic and drag it to a target place in the list and
press CTRL so that the cursor changes to ‘+’. Drop it and it will
create a duplicate along with its children.

Or,

2) Copy the topic to the clipboard and paste.

The nice thing is that these methods allow you to copy topics to another document too. Also, the child topics are automatically copied with the parent.