Should I use canonical tag in these cases?

[GoMentor]

Should I use canonical tag in these cases?

• On the page itself (with the tag pointing to itself)
• On pages that doesn't have duplicate versions

[LoganRay]

Good question, one that I don't feel like gets addressed enough.

Yes, you should always include self-referring canonical tags. There's a few reasons for this, but primary it helps one version of your URLs get indexed. Here's a handful of cases where they're helpful:

1. Some CMSs create URLs that are case-sensitive - i.e. URLs will resolve at /Some-page, /some-page, and /Some-Page
2. HTTP vs. HTTPS - if you've gone secure, self-referring canonical tags can help search engines learn your new structure and drop HTTP URLs from the index quicker, or at least prevent both secure and non-secure from being indexed
3. Absolute/Relative links - some development teams prefer to use relative URLs in links when working in Dev and Test environments, this is helpful for preventing unwanted Dev/Test URLs from getting indexed, but isn't ideal for SEO. This is where self-referring canonicals come in.
4. WWW vs. Non-WWW - another safeguard to prevent indexing of both versions, even with redirects in place, it doesn't hurt to have a fall back
5. URLs with Parameters - If your site appends parameters to URLs for any reason, self-referring canonicals will prevent indexation of /this-page?q=123

There's probably other reasons to add to this list, but these should be compelling enough to go ahead and add them.