How to get the unique categories from a list

Question

I have a set of FAQ entries. Each entry has one or more categories attached. To be able to filter entries related to a category I need a list with unique categories.

Using my solution I'm getting all (some double) categories.

   {% set entries = craft.entries.section('faqs') %}
   {% for entry in entries %}
        {% for category in entry.faqCategory %}
             <li><a href="#{{ category.title }}" class="filter" data-filter=".{{ category.title }}">{{ category.title }}<ins>&nbsp;</ins></a></li>
        {% endfor %}
    {% endfor %}

Answer 1

Maybe not the most elegant, but something like this should work:

{% set entries = craft.entries.section('faqs') %}
{% set usedCategoryIds = [] %}

{% for entry in entries %}
    {% for category in entry.faqCategory %}
        {% if category.id not in usedCategoryIds %}
            <li><a href="#{{ category.title }}" class="filter" data-filter=".{{ category.title }}">{{ category.title }}<ins>&nbsp;</ins></a></li>

            {% set usedCategoryIds = usedCategoryIds|merge([category.id]) %}
        {% endif %}
    {% endfor %}
{% endfor %}