Duplicate content is a conundrum at best. if you look at:
https://support.google.com/webmasters/answer/66359?hl=en and then scroll down about 3/4ths the way down you get to "
Duplicate content on a site is not grounds for action on that site unless it appears that the intent of the duplicate content is to be deceptive and manipulate search engine results. If your site suffers from duplicate content issues, and you don't follow the advice listed above, we do a good job of choosing a version of the content to show in our search results." It wont hurt you, but if we deem it a scam it will.. and if you do have duplicate content we will select one for you.
I will say i play a bit cautious in terms of what I will do with SEO. When I am working with clients... I am even more so cautious. When Google says it wont hurt.. it can hurt and if all else fails we will list what we want to... I take notice. Its not just Google either.. there are reason that there are tags that are now for the most part universal. canonical linking, no-index, Author tags.. these are all methods that would be prefered to identify duplicate content be it on your own site.. or out on other pages.
So when we start talking about writing a piece of content. putting it on your own site,and then saying "Hey I should share this on other sites" there are a few things i do. I ensure there is an author tag on the page or post o my site. I ensure that the page is indexed on my site prior to sharing. I ensure there is a link back to that specific page on the other sites I may be posting this content. ALL of this is to ensure that Any search engine knows Whos content it is, where it came from and the fact thatthere are now multiple copies running around.
So now we can start talking about the reason for doing this... most SEO guys are doing this to create backlinks.. its kind of silly really if you think about it. If Google is going to index only 1 copy of this content,obviously you want to ensure it is the copy that is on your site.. right? So by sharing content across the web in the hopes of a backlink.. will it really count if the content is deemed "duplicate" The answer is "It Depends" and how exactly it depends, I still haven't figured out and may never.
The reason I use duplicated posted content really has nothing to do with SEO.. but has everything to do with direct flow of traffic into groups of users I would normally have the opportunity to share and communicate with. If they are on Site Z and see an article I have written they MAY click on the link ( that I ensure is on the page ) and check out more of my content on my site, Site A. So SEO aside... if the intent is using this content to increase your audience reach, I would say follow the rules and have at it.. If SEO is the intent.. again I would say follow the rules and it may or may not work in your advantage... and if you don't follow the rues, YOUR content may be helping the site you shared on and NOT yours.