Why are there duplicate pictures in the preview?
If your photos appear more than once on the preview, it might be because they are located in multiple facebook albums at the same time (for example Mobile uploads, Profile pictures, Holidays 2015,...).
If you want to remove them go to the edition page and scroll down to see the "Edit content" button, click on it and uncheck picture you don’t want.
On the left side, you can also remove a complete album like the "Instagram photos » album.
Or delete them directly on Facebook