Friends who subscribe to my main RSS feed may wonder why some posts pop up again after a day or two, or maybe even a week after they've been posted. This is mainly for two reasons:

1. This RSS feed is an aggregator, and sometimes it aggregates by simply scraping another weblog's front page, not by checking its RSS feed. When I add these scraped posts to the main aggregate RSS feed, I include the little "Comments: 0" bit, so that when somebody comments on a post that is on the front page of the aggregator, the comment bit will change to "Comments: 1" and it will show up in the RSS feed as a new post.
2. I'm a stickler for web standards, so sometimes when a post breaks the validation of my homepage, I will go back and fix the mark-up. If this happens after your RSS feed reader has downloaded the feed containing the non-validating post, then when youre feed reader downloads the new feed with the updated, now-validating post, then it will show up a second time as a new post.

Stuff like this happens when you DIY.