Say I have a forum website and one of the user posts the same thread on my forum and other similar forum websites? Will google penalize my website if this happens a lot? And what if the same situation occurs with the thread replies too, like exact same thread?
Google filters duplicate content. The domain with the most authority will be displayed in the search results.
There is a reason you know forum links are extremely hard to rank. Because forum comments & posts are bound to have to some content which can be categorized as 'duplicate' content or something like that. Search engines such as Google mostly filters out or ignores these type of content.
Google know when the content page is indexed on their database, so if there are duplicate occur on the web it will consider as duplicate and will be ignored for ranking page.
There will always be duplicate content, especially when you are dealing with a forum. People like copying stuff an auto-blogging, sometimes just to test. But it is hard to come by an authority or a serious site doing the copying. nobody would really create a website with the sole purpose of copying other sites. So, in many cases it is a small site which would just disappear. Furthermore, the first one to be indexed on Google will have the upper hand, and the other one will be up for a short period. It wouldn't really give me sleepless nights
In today's world, there are so many tools available through which one can easily find that content is copied or not. Yes, we can't deny the fact that people do copy contents but on the other hand, having advanced tools this copy system is prevented to some extent.
It will be treated as duplicate content, Google most certainly wont be able to determine that the content is user generated.
It will ban your website if you post duplicate content in your website. be aware from duplicate contnent.