As far as I know it's not even possible to have a duplicate domain
I assume you're referring to sites like blogger.com and wordpress.com and the thought of switching to your own domain?
Google trusts your site more if it is on a public blog system which polices itself but there is also a rankings ceiling that is lifted when you switch to your own domain so definitely buy your own domain if you can afford it. $10 a year isn't overly expensive and you can point that domain at your blogger or wordpress blog so there are no hosting fees.
Recently, Google's algorithms have been favoring brands over exact-match keyword domains, not exactly "devaluing" EMDs - the way, in my opinion, it should be. The penguin update hit a variety of SEO strategies hard, however, and that's because they were already teetering to begin with. SEO is supposed to be timeless. There won't be exact-match domains forever and Google is aware of this. I would go for the branded approach, but you can experiment and see what works for you.
I prefer brandable domains over exact match ones but the domains with keywords in them are far from useless. Its just that google stopper prefering them over anything else. Keyword domains are good for 'micro niche' sites, but I always use brands on my serious, long term - or at least intended to be long term - sites. Its just easier to build an audience if you visitors can actually memorize your URL
As far as I know Google does not rank better keyword domains just for the name of the domain. You can have a branded domain without your keywords in it but if you have some SEO friendly content you should see it ranking well.
I heard that in recent Google updates, they 'nerfed' EMD's, and they are not as effective as they were in the past. They would originally think of them as the website they needed that has quality content, good interface, when after all, they were poorly designed and had bad content. EMD's are still useful since the domains are easy to remember, but the thing that you should be focused on is what Google wants; quality content.
It's true; EMDs were supposed to have a lower weight carried with them and their PR, as the search engines accidentally gave them preference over more organic, unique, "brandable" domains, but many still argue that, despite this. It is more of a flaw with the way the algorithms work than a knob they can just "fine tune" when it comes to keyword domains, so to speak. They also do not want to penalize small businesses who flock with domains like (insert location here)pestcontrol.com just because they have the keyword in their name. They are attempting to find some sort of balance between keyword and unique domains that they will achieve at some point in time, just not so much right now.
However, consider this scenario. There are thousands of domains with the keyword "dating" in them, either followed or preceded by a niche keyword. If one were to attempt to get an EMD in this niche, the competition will be too high for you to see your domain anywhere near the first ten pages, except for certain keywords. Even then, it will only happen if the site is very niche. This is an example where EMDs can actually be disadvantageous.
For the future, it would be much better to have unique domain. In the long run, they are likely to outrank EMDs, just not at the current moment.
If you've a powerful product, service, or a message to spread out to the world and have a powerful SEO strategy to go with it, then go for the branded domain name. SONY, Samsung, eBay, Amazon, etc. are top examples of a branded domain name, because they have powerful products, powerful services, and messages that have attracted millions of visitors.
Domain names are always unique. There cann't be two www.google.com; It is said that domain names don'o affect SEO, but practically at present it do. If domain names are registered with same keywords then there are more probability of improvement of the domain in SEO. If Google changes the algorithm of panda and penguin then things will change too.
Keyword domain is , eg: you are manufacturer who produces steel pipes. Now if your domain name is www.steelpipes.com then when people will search in google with 'steel pipes' there is high possibility that your website with domain name www.steelpipes.com will appear on front page of google. This type of domain names are known as keyword domain. There is nothing as branded domain.