I'm sorry, but I'm not a SEO expert, so I can't say an exact answer here. However, I can say that it's not required to have a duplicated cross-posted content in Blog and in Forum. You can change the content of first post content of cross-posted topic (make it just an intro of the main blog post, or something else), you can disable Forum to blog cross-posting and only enable blog to forum cross-posting (move some engagement from blog to forum), so all topics and replies will be unique in Forum topics and in Blog Posts. Also, people will start opening new topics and new discussions, this will bring more unique content for website SEO.