Duplicate elements being displayed with relatedTo param

[connor-baer]

Description

There seems to be an issue with the relatedTo query param in Craft 3. When I try to get all categories/tags related to the entries from one section, the categories/tags are displayed multiple times. Specifically, the categories/tags are displayed for each entry they've been assigned to. The desired behaviour would be each category/tag is only output once, right?

Steps to reproduce

1. Create a few entries in a section and assign categories to them. Make sure to assign one category to multiple entries.
2. Put the following code into a template:

{% set posts = craft.entries.section('sectionHandle').limit(null) %}
{% set categories = craft.categories.relatedTo(posts) %}

{% for category in categories %}
    {{ category.title }}
{% endfor %}

3. The title of a category will appear for each post it's been assigned to, i.e. multiple times. It should only appear once, right?

Additional info

• Craft version: Craft 3-beta.12
• PHP version: 7.1.4
• Database driver & version: MySQL 5.7.17
• Plugins & versions: Element Api 2.0.1