Well for one you can't break the law, meaning stealing copyrighted content. You can syndicate content though, and there's a ton out there that you can, even if it's stubs and not full articles. There's no specific rule saying you have to write all the content yourself.
The major rule with AdSense is you can't seem "MFA", or "Made For AdSense". That's vague, but it basically means the site is made just for AdSense revenue. It's a tightrope, but if your content isn't original, your aggregation has to be. It has to seem like you're putting effort into really selecting articles, categorizing them and posting them for the sake of creating a good site to read as opposed to filling a site any way you can to run ads.
If you build one site that is nice looking and has a reasonable amount of unique (not in the sense that it's original, in the sense that you're not just grabbing 100 articles that basically say the same thing) content you'll probably be accepted. Create a facebook page for it, a twitter, disqus comments, etc. Even get some conversation going in comments sections or get some likes to the facebook page. If you do that you can set it apart from all the MFA sites.
Once you're accepted you can be looser. The big thing you need to know here is you can't buy traffic to your pages. That's arbitrage and they'll decide beyond any doubt that it's MFA. Don't do it at all. You can blank referrers or try to filter it somehow but AdSense is too sophisticated for that shit to work.
Now the thing about the duplicate content is that while it's legal, the Google organic search won't like it. You won't get penalized, but more often than not the original source will be displayed instead of yours. It's not illegal, not a penalty, won't affect the overall heath on your site and won't be seen as a violation of AdSense TOS. You'll just find SE traffic is harder to come by. At the same time, SE traffic isn't everything these days. If you can, say, get subscribers to your site on Facebook and get content shared it won't hurt you one bit if it's duplicate. Look at sites like upworthy or even buzzfeed to an extent.