Reactive chapter is duplicated in the side menu

[kawamataryo]

Hi.
Reactive chapter is duplicated in the side menu.
Is this as expected?
If not, I'll fix it, but which is correct, Internals or Advanced Guides?

Config is here.
https://github.com/vuejs/docs-next/blob/master/src/.vuepress/config.js#L48
https://github.com/vuejs/docs-next/blob/master/src/.vuepress/config.js#L71