I have a question regarding canonical-tags where im not pretty sure. I have a main-website and a main-shop and a brand-website with its own shop. The products of one brand are in the brand-shop and in the main shop with mostly the same information. Should i use canonical-tags on the brand-website to show that the origial source is the main-shop? Does this affect seo in a bad way when i have the products on both pages or can i just leave it without a canonical-tag and without any consequences?
The scenario you are describing is generating duplicate content across multiple domains.
Duplicate content generally refers to substantive blocks of content within or across domains that either completely match other content or are appreciably similar. Mostly, this is not deceptive in origin. (link)
Although duplicate content doesn’t cause your site to be penalized, Google will try to determine the original source of the content and display that one. It's a better option if we help Google by using the
canonical tag to select the option we prefer as the main one.
Let’s put this to bed once and for all, folks: There’s no such thing as a “duplicate content penalty.” At least, not in the way most people mean when they say that. (link)