If you have a legitimate reason for using similar or duplicate content across different sites or domains, there are ways of managing it so as not to negatively affect your SEO. Janne is going to tell us how.
“Some sites need to have duplicate content across different sites – for instance, if migrating to a new site domain.
If two sites need to exist, ensure you reduce in-site duplication where possible, and put up 301 (permanent) redirects. This will ensure future work is kept to a minimum and visitors and spiders will be redirected to the newer site.
If you’re not able to remove content or set up redirects easily, you can use the rel=”canonical” link element (more detailed information can be found from Google) across the domains to specify which domain is the preferred one and which should be used for crawling an indexing.
If you use the rel=”canonical” link element, ensure you also create an updated sitemap to identify preferred URLs. By doing this correctly you can boost your SEO link power to your preferred domain. Your older domain will obviously lose SEO gravitas, but this shouldn’t matter if your main site is up and running correctly.”