I think a policy of linking material from elsewhere is the way to go. It's not plagiarism and it doesn't risk violating any licensing by duplicating to a new medium, and most of all it is honest about intent. That is to say, you have linked it because you are interested in discussing the content with the Steem community.

Personally I recently posted a link about tech in the distributed web summit which was somebody else's blog post, and I hesitated to do so because I didn't want to claim credit for it, I just thought the community would be interested (turned out they weren't, no comments at all!) To my mind, if I could have disabled payout for the main article other than that generated through comments I would have.... My contribution would only be a contribution if the Steemit community cared to discuss it.

In that example it's all pretty moot, but that's how I felt about it. I think a bot that comments on articles that it detects as being duplicate, linking the source article is probably a good idea - it doesn't enforce anything on the community but does steer it in the direction of acting honestly and safely.