There are several different URLs that can be used to access either the list of tags or individual tags which list the items that have been tagged (depending if you have the SEF plugin enabled, etc..)
The tag component should add a canonical link which points to the preferred URL of one of its pages (probably based on a menu item that exists). This way Google will not get confused about which URL to use and the page will not be hit with a possible duplicate content issue.
This will also help in situations where someone decides to add/update/change the SEF plugin they are using which could result in different URLs being generated for the tags component.
For more information on Canonical URLs and how they should work see: https://support.google.com/webmasters/answer/139066?hl=en
Add to the head of the page the proper canonical URL. Example:
<link rel="canonical" href="https://www.yoursite.com/tags/tag-alias" />
No Canonical links are created at this time by the tags component.
Articles have a similar issue which has been fixed with this pull request #18318
No Code Attached Yet
|Category||⇒||com_tags Feature Request Router / SEF|
#22589 is some what related along with issue #18219. What this feature request does is add a Canonical URL within the html source that points to the correct URL for the content. This is used by search engines like Google so they know which url to display on the search engine result pages in the case of when more than one url will display the same page. (and to avoid duplicate content issues)
I don't really understand the Joomla Dev Team sometimes. Canonical URLs is such a basic and IMPORTANT part of SEO. To not have it implemented for all the core components is crazy especially with the way the Joomla Router spits out different URLs for the same page.
Joomla does a lot right but this is one thing they need to fix (not just for the tag component).