photos have duplicates on iPhone but not on the PC

Why do some photos have duplicates on iPhone but not on the PC?

I have the iPhone 3GS and synch it on a PC, XP.

Regarding photos I have them under "My Pictures" and are listed by folder e.g. a
vacation, family members, etc.

I noticed that several of my photos in some folders had duplicates on the iPhone
but not in the computer file, but most of the photos did not have duplicates.

I couldn't figure out how to delete the duplicates on the iPhone so I unchecked
all the photo folders in iTunes and did a synch. Then I closed and reopened
iTunes, rechecked all the photo folders and did a resych.

Now it looks like even more photos are duplicated on the iPhone, but all, and
only in certain folders. The PC folders remain constant with one of each photo.

Please help. What could be causing this and how can I get rid of all the dupes
on the iPhone?

iPhone Photography
When I set iTunes to sync photos from a location, I have no options to pick and choose what subfolders to sync, it just syncs photos in all subfolders of the selected location. The only option I have to sync some but not all is if I instead set it to sync albums from a supported photo manager application (Photoshop Elements in my case).

This is also in Windows, using iTunes 10.