Well, here's a wrinkle that I just learned. (You'll probably have to read this a few times...) I'm using Ozh'S YOURLS as a locally installed shortener and it properly generates a shorturl and tweets a new post, but two tweets show up. Researching, I find that the second one is the post pulled into the Tags Blog and IS readable there (news to me) BUT it has a different shorturl in the WP default permalink style in the shorturl header, even though the second YOURLS shorturl is in the Tags Blog post Custom Fields AND the permalink structure is set to /%category%/%postname%/.
The YOURLS shroturl 301s to the above permalink version but that non-YOURLS shorturl loads the same post and doesn't 301. So, we have what looks like two duplicate posts in the Tags Blog having different URLs.
There is a canonical header that points to the correct source blog permalink but canonicals aren't supposed to point between domains.
The kicker is, YOURLS WordPress to Twitter plugin is not activated on the Tags Blog, so a second shorturl from it is weird. If it were, I could see some interaction going on but the interjection of a default permalink as a shorturl in place of it seems even more strange.
I'm on a subdomain install and hadn't considered the sitemap issue. I'd wager that this situation may make that even worse.