Hello,
I use the famous vbulletin forum system for my forum. In their 4.0 version, you have the ability to promote a post into a blog or article.

Now does this affect seo negatively because wthere will be a duplicate content?

Duplicate content always has a negative impact on any site which uses it. As you are fully aware of duplicacy, you can manually review the articles and let them go through a " copyscape " test to check the percentage of copied content.

I agree with M.Rebecca, always content is king.