From another thread I found you can set the canonical URL in the advanced settings. By doing this and entering the full URL it immediatly fixed the problem. All the Yoast tags are correct.
I think this means that it is YOAST that is adding the superflous domain. I now have the correct
<link rel=”canonical” href=”https://www.domain.com/cornwall-visitor-information/royal-cornwall-show-2015-video/” />
instead of
<link rel=”canonical” href=”https://www.domain.comhttps://www.domain.com/cornwall-visitor-information/royal-cornwall-show-2015-video/” />
This domain duplication is only incurring in the YOAST generated tags. Or at least only n the tags immediately under the YOAST header. I do not think another plug-in is interfering with the tags generated by YOAST if it was I do not think adding the canonical URL in the Yoast POST advanced settings would fix it.
It is NOT an issue of duplicate tags but of corrupted YOAST tags. Great that I have found a fix for all new posts but I really do not want to go back through fixing every post and page in the site!
I am not clear where we report this as a bug. Can you help?
I think your issue was similar but not the same I had that with a theme Social settings that were adding the tags which were empty. I disabled the themes ability to set OG tags and that fixed the duplicate issue.
Thanks again for all your help.