Took on a website`s SEO today that appears to have had https running alongside http pages since 2006. The canonicals were pointing to the http pages though. I can see this using WayBack Machine and viewing source.
However a well meaning designer recently saw in about Jan this year that the site had http and https alongside each other and redirected the http to the https and changed the canonicals and the sitemaps to https. However, he did not sort any of the internal links or add https to GA or GSC and will not as he says he did the first bit as a "favour" to the owner to avoid duplicate content.
My question: Would there have been duplicate content if the canonicals were http?