I have seen this problem popping up all over the internet as others begin installing 1.6 for the first time. Here's the upshot of my research.
There's a problem in the code, yes. However, I've tried quite a few of these "fixes" and the bottom line is this, they don't "last." You can clear your caches until you are blue in the face and it's still happening. For the record, I performed a clean install with no sample data. I created one article for the front page with an image. That was the only article in the database. I left it and came back the next morning. I was seeing double again.
After about a month, non-stop, tracking down this issue, it "appears" to me this problem is based on some sort of timer or related to the date somehow. It's fine tonight and tomorrow you see double. After about 8 installs with varying degress of articles and custom information which all net the same results, I give up.
I'm waiting on the Joomla team for an answer before I start making any more changes to the core files. Otherwise I may be opening up some security issues on my sites in ignorance, as well as additional problems I'll have to either take note of or backtrack on later.
It seems foolish to me to assume all the "seasoned" site designers having the same problem are making fundamental mistakes (i.e. forgetting to clear cache, check settings) which is creating the same problem. There seem to be many "fixes" but no cure for the double posting issue at present.
I strongly caution everyone except the most proficient on php codes and interrelational databases to stop making changes to the core php files until the developers at Joomla have had an opportunity to fix this double-posting quirk. The developers are usually pretty quick to issue a fix. Since I know this bug was submitted to the bug squad, I'm sure we'll have an official fix soon.
Carol