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Yes, the Guest post is still working for link building to do website SEO. I have done it on my Impressions Kitchen project, you can see here website.

We had this issue with products which we sell on two different websites, that Google would rank one or the other, randomly swapping them out for each other in the rankings. We used cross-domain canonicals to solve this and it has fixed the issue for us, i.e. the website we want to rank now ranks for these products, the other one doesn't. If you use canonicals, it will most likely mean that only the main page will rank (the canonical URL).