Many so calles "seo gurus" telling you quackery like;
double content is bad- but not mention that it is only bad on your one domain
multiple domains harm your seo- which is not true see all the big boys like Hearstmedia, Gawker, Trullia and many more...etc,etc,etc...
keyword cannibalization harms your seo.....
They not can explain you:
how get multiple serp listings or dominate first page with or without multiple domains.
Google telling you same big lies / not so lies but in very sophisticated way that they now showing only two urls form one website.
All of that above is not really true.
Check Zilla, Bloomberg, Hearstmedia, Gawker and there are more of them....
they ranking multiple times in the serps or on first page or having multiple domains (it's so bad for seo right?)
using many times, the same title and meta descriptions on their other domains (with canonical link)
they doing keyword cannibalization
But google and seo gurus telling you that this will harm your seo.