I'm currently working on SEO product content and I'm looking to add canonical tags to 2 product pages to a 'master' product page.
I've added rel=canonical tags into the HTML of the two non-canonical pages pointing towards the lead page but when I scanned the page it says I have duplicate canonical tags.
Help would be much appreciated!
It looks like you've placed an extra canonical tag inside the product description field. This is inside the <body> not the <head>. So it's in a non-standard location within the HTML and is a duplicate.
Unfortunately Shopify doesn't provide a mechanism for canonical overrides, so there are some workarounds. If you want to change canonicals you'd normally need to edit the theme code. Usually you'll find this in the themes.liquid file, however not all themes are structured the same.
What I usually do is setup a metafield like seo.canonical and create logic in the themes.liquid file to check if that field exists for a product then replace the theme default canonical with the custom canonical. Ie use it for custom canonical overrides.
Also - just a note of caution if you get hands on with theme code it's pretty easy to learn, but also easy to break the site with a typo. Shopify has file rollbacks and lets you download a theme as a backup, not a bad idea if you're unfamiliar.