Can Anyone Answer My Question?

I have been doing SEO with this site [You must be registered and logged in to see this link.] for a week now..

Today, I have recieve a message from my client that he purchase another 2 domain with the same site.

GUITARSLOWDOWNSONG.COM
SLOWDOWNTHATGUITAR.COM

I would like to know if Google will penalized for having multiple domains..? ... and duplicating content? I have made a little research on this but I find different results..
Some say yes and some say no . . . . which is which?
Please i need some advice from you guys... is it effective to SEO multiple domain? or if not, what am I going to do with this? And what are the benefits or risks of this..
Lastly, how to do effective SEO for multiple domains?

Please help me.. Thanks.

I don't know.

But in my wild guess, it won't.

There are a lot of tiny URLs out there and they have no impact on the original domain.
But for sure, backlinks to each domain have different value / score.

Dude! if your content of site are same then you have to redirect domains to your master domain which you already optimizing and if you submit all domains to google and do seo for that it will be spamming and google will block your results. In my opinion you have to redirect your other domains and or you can read here the best solution for duplication

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Regards,

Mike John
SEO Expert