List entries related to several categories
Not sure if this is still what you are after (I removed a lot of parameters), but basically this gets the one (!) category with the unique (!) slug you request from the URL and then lists the entries related to it:

{% set categorySlug = craft.request.getLastSegment() %}
{% set category = craft.categories.group('compositions').slug(categorySlug).first() %}

&lt;h1&gt;{{ category.title }}&lt;/h1&gt;

{% set entries = craft.entries.section('contenuCompositions').relatedTo(category) %}

{% for entry in entries %}
    {{ entry.title }}
{% endfor %}