It seems that some of my posts are appearing on Planet Debian multiple times, and others not at all. I think the problem is a strange combination between subtle bugs in Planet and Serendipity. The SF bug report for Serendipity is here. I think the workaround should fix this.
Either that, or Planet is somehow taking offense as the <slash:comments> tag that Serendipity is putting in there.
Sorry about that — hopefully it is fixed now.