I found the solution for not getting penalized for Duplicate content while still using Posterous.
Option 1:
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Basically edit your Posterous theme and add:
<meta name=”robots” content=”noindex, follow”>
in the <head> section.
instructions found on:
Don’t let Posterous ruin your blog’s SEO | Vertical Leap Blog
Option 2:
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Google now does canonical tags for cross domain legitimate Duplication:
Official Google Webmaster Central Blog: Handling legitimate cross-domain content duplication
Haven't tried option 2 yet.
i use this on
iPhone Tutorial Videos as I found if I don't use Posterous, i just don't post often enough...