We have a family of sites (about games) with shared content. Each site has its own top level domain, and most content has a "home" domain, but all content is accessible on each domain. This allows a user who is logged in on, for example, the board game site, to page through their new subscribed content and see pages about RPGs or video games (content that is based in another of our domains) without having to jump to another domain.
I am concerned that this duplicate content will be used to penalize us in search engine rankings. Canonical links do not work across domains. Google recommends using 301 redirects to force all users to a single domain for a particular page, but we do not want to do that because we don't want to force users off their preferred domain. In addition, we have other content that genuinely belongs to multiple domains--lists that might include games from multiple domains,for example.
How can we continue to show our content in this way, without being penalized for having duplicate content across domains?