I have a main website but I have copied my website to 5 sub-domains each targeted to a different country. Each subdomain's content is same, just a variation in prices, currency and language.
Someone told me that it will cause duplicate content, but if I set canonical URL of main website where I have published this content first then it will not cause duplicate content.
When Google encounters duplicate content it usually just picks one copy to index and ignores the rest. A canonical tag would let you choose the one that Google indexes.
However for site copies targeted at different countries, Google specifically allows duplicate content. Google wants to index all the copies and show them in the search results each to people from those countries. This is because Google recognizes that it is good user experience to differentiate languages, prices, and taxes between different countries.
You should NOT use canonical tags for copies of your site targeted at different countries. Rather you should use hreflang tags to tell Google about the different versions. See Tell Google about localized versions of your page - Search Console Help
Sources: What is duplicate content and how can I avoid being penalized for it on my site? and How should I structure my URLs for both SEO and localization?
As stated by MOZ - "A canonical tag (aka "rel canonical") is a way of telling search engines that a specific URL represents the master copy of a page." This works for external sources as well.
