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You have to do page by page like Nigel suggested that is the best way to do it and keep them in place for ever!.

Useragent:*
Disallow: /*?expand=

This should work put it in your robots.txt

I have an ecommerce website where some pages link to a product page with a different URL.

EXAMPLE:

1: /category/product1.html (not indexed by Google) with canonical pointing to product1.html

Other page link to the product like below.

2: product1.html (indexed by Google)

Now the question is, does 1: pass any link juice to product1.html or not? Is it worth to change everything and link only to one URL? My site is running on Magento!

If the look and feel is different on each site and each has it's own content it would not really matter. Just be careful interlinking those site with each other. If you can assign each website with their own IP that would be the best!

Redirect every page to the relevant page on other website and you will pass link juice and get higher Authority. Keep the redirects in place and you will see results within 2 to 3 weeks.

Yes correct the blog is site.com/blog and has high quality content with many high ranking pages and high quality score. I will clean it up and try to maintain it again as the blog has not been looked after for a while and can be a good source for extra traffic.

Many thanks for all comments it really helps!

I have a huge blog which is connected to a company website. This blog is full with dead links and spam comments. Is it worth to clean up?

The website itself ranks very well in search engines. I'm amazed with all de spam that it still ranks. Do you think by removing all de dead links and spam the site will improve ranking in search results?

If there is a no follow tag it would not matter. Google will ignore those links anyway if no follow tag is insert. If you feel not comfortable with it you can always use Google Search Console to disavow links like Nigel explained.

Content inside an "accordion" module can be seen by Google for example if read more button is clicked to display more text. This is not hidden content and do not have to worry.

If they are linking with no follow tags to your site they will not harm you. No worries!

Redirect every page to the relevant page on other website and you will pass link juice and get higher Authority. Keep the redirects in place and you will see results within 2 to 3 weeks.