Been through this many times, I do have in my forum rules to link sources of articles etc. However worst case scenario is someone asks you to remove that article and content. As for the 2nd part..if your rules say source the article etc and the user doesn't it's on them not you. ( Actually did some research on all this once).
thats not really the worst case. The worst case is that you never hear from the author, so that may sound good, but google
will see that you have duplicate content, source link or not, you are not adding anything new, just re-hashing content....
so rather then getting the best rank you could get, you end up with a lower ranking, or it just drops out of site all together...
I have had duplicate content before, some posts never got indexed at all, other posts took a noise dive in rankings....
One month I have a site making decent income, and the next month the math was easily to count, all zero's....
It can take sometime to get into the mix of the traffic, and it just takes google 1 min to remove you from index.....
The worst stories I hear is when people say "someone stole my content, now they are
ranking higher then I am..." They are not ranking higher because they stole your content,
they are ranking higher simply because they have more authority in their site, and
they are simply using your own content against you. Google gives much more authority
to sites that are in the position of authority, then sites that struggle to get there, and to make things worse, google often turns a blind eye, and authority sites can get away with
things that non-authority sites would never get away with in the eyes of google.