EngineHandler doesn't raise exception when duplicate aliases are registered

[Preston Timmons]

The django.template.utils.EngineHandler has a check to raise ImproperlyConfigured if multiple engines are specified with the same alias:

​https://github.com/django/django/blob/master/django/template/utils.py#L72

This won't ever raise an error, though, because configuration is stored in an OrderedDict. Duplicate aliases simply overwrite previous ones.

Curtis ran into this problem when using settings like this without the NAME option:

TEMPLATES = [
    {
        'BACKEND': 'knights.django.KnightsTemplater',
        ...
    },
    {
        'BACKEND': 'django.template.backends.django.DjangoTemplates',
        ...
    },
]

[Aymeric Augustin]

How embarrassing.

[Aymeric Augustin <aymeric.augustin@…>]

In 1563b89:

Fixed #24685 -- Fixed check for template name unicity.

Thanks Preston Timmons for the report.

[Aymeric Augustin <aymeric.augustin@…>]

In 1f0a6082:

[1.8.x] Fixed #24685 -- Fixed check for template name unicity.

Thanks Preston Timmons for the report.

Backport of 1563b89 from master