why do I get duplicates of every photo on photostream?
There are many possible reasons.
One is you set Settings > Photos & Camera > Keep Normal Photo = "On".
Another is you already synced your iPhone and have the photos in Camera Roll on the iPhone and whatever Photo storage place you use on your computer.
You mean that Photo Stream has two of every photo?
Do you have HDR turned on on your Camera (top of screen - HDR On?) If you do, then that is why. With HDR, you get the original photo and an HDR (High Dynamic Range) copy of the photo, which balances the light and dark in the photo. You can choose either one, then delete the other. Or, if you prefer, you can tap on HDR On, and turn it off.
Cheers,
GB
Photostream
