Thank you! And yes, I have the same problem with blogger, I’m just deleting the second post manually for now, but anyone reading my blog through an RSS feed is going to be confused!
I tried emailing posterous to ask for help with that, but apparently my question confused them, since the answer that came back was less than helpful - they said you can send to posterous@xx.posterous.com and it will only post there - except that I find it then just doesn’t post at all, anywhere. Ah well, teething problems…