That’s not what’s happening, when you visit /contacts--/ it doesn’t return a 200 code, but instead it returns a 301 redirect code. The browser then follows this redirect and heads to /contact and it’s /contact that returns 200 code.
This is because /contact is the canonical URL of that page, and WordPress redirects to canonical pages out of the box for improved SEO. It should also be adding a meta tag to the header containing the canonical URL to avoid duplicate content penalties.