Points of clarification since you guys seem to miss it.
If you want to 100% duplicate the SAME CONTENT with ZERO changes between them, no matter WHAT you use, it's a bad idea. Don't do it.
If you want to duplicate content in multiple LANGUAGES or with customization for locations, that's different and it's fine, and yes, Multisite is great for that.
What I'm saying is that, in general, duplicating content does the following:
1) It dilutes your brand - People have TOO MANY places to go to find you.
2) It dilutes your SEO rank - Search engines have too many places to find you.
All that goes out the door, though, if you want multiple languages. THAT is a GREAT reason to do it :) And you DO NOT lose SEO.