Last week I noticed that some contacts were repeated in my iPhone contacts. That's awful. It makes the phone suddenly very hard to use.
I did a little reading, found some stuff about various levels of synchronization. I have a Mac OS X 10.4, plus 10.5, plus the iPhone, plus my gmail and yahoo! mail accounts.
Something's not working. I cycled through just about everything until I read the MobileMe news, with this article: MobileMe: Service Updates as of December 15, 2008 .
Essentially, sync is broken and fixed in 10.5.6. One OS upgrade and things are fixed.
Currently, I have the Yahoo! and gmail sync off. Now I can try turning these back on.