Can't add directories from plugins if partially string matches existing values

[wardi]

CKAN version
1.7 or so

Describe the bug
If you add a served directory that partially string matches existing values it will be ignored

Steps to reproduce

if old_ckan:
    add_template_directory(config, 'templates-for-old-ckans')
add_template_directory(config, 'templates')

Expected behavior
when old_ckan is True I expect both template directories to be in the template search path

Additional details
only the /path/to/my/extension/templates-for-old-ckans appears in the template search path