Postname on unique permalink structure appends “-2” for a custom post type. How can I get this to stop happening?

By default, WordPress disallow 2 identical slugs because if you change your permalink structure for custom structure, you should be able to have in your case:

• https://example.com/saint-louis/
• https://example.com/saint-louis/

And you can understand, it’s impossible.

The only way is checking by code, you are not in “custom structure” and then save in db with the WordPress method (adding “-2”) and then add a rewrite rule, or find a proper hook to remove it before use in front-end.