Could it be that, due to the need to all have the same "design/theme", google interpret them as duplicates? I also see that rel="canonical" in https://develo

Could it be that, due to the need to all have the same "design/theme", google interpret them as duplicates? I also see that rel="canonical" in https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls https://developers.google.com/search/docs/crawling-indexing/... (which is the first time I hear about): is it kind of necessary?