Question about structure of postfix_sender_canonicals variable

[ghost]

The variable is used by the template templates/mapfile.j2. The Template has following structure:

{{ ansible_managed | comment }}

{% for mapping in __postfix_mappings %}
{% for key, value in mapping.items() %}
{{ key }}    {{ value }}
{% endfor %}
{% endfor %}

The question is, why is it a list of dict keys, which should be unique?
Currently you could do something like this:

postfix_sender_canonicals:
  - root: test1@example.com
  - root: test2@example.com

Which would result in:

root      test1@example.com
root      test2@example.com

If the variable would be a single dictionary, such thing would not be possible because a dict can not have two keys with the same name.

So the question is, why is it implemented as a list?