This add-on allows you set set a canonical tag for all of your tags.
This screenshot will make more sense:
You select the canonical tag, and any request to the non-canonical tags page will be redirected to the page of the canonical tag.
There is also an option to automatically change any non-canonical tag to the canonical version when it is used in content - this is enabled by default:
You can demo this in the test forum at XF Liam - the tag 'xf' has been canonicalised to the tag 'xenforo'.