Personal galleries allow you to organize your photos (type, camera, lens, etc.). If you use the Member's Gallery the images will still show up as your images. When someone clicks on the 'My Gallery' link in your signature, your Member's Gallery images show up along with your personal galleries (provided, of course, the link points to your RFF gallery).
For instance, I no longer have personal galleries, so when you click on my RFF link below, you'll see all my images (I only have one page worth of images), all placed in the Member's Gallery.
But if you click on
graksi's gallery you'll see he has one Album (personal gallery), and a bunch of images sitting in the Member's Gallery.
Duplicates occur when you upload images and choose to upload to more than one gallery. If someone clicks on the Recent Uploads (to review what has been uploaded) the duplicates show up.
The problem with duplicates (besides the fact that they clog up the Recent Uploads) is, as Einars stated, if someone wants to comment on your image, which one do they comment on? Comments for your one image could end up spread across several duplicates.
Also, the Member's Gallery is the only gallery that I'm aware of, that has fields for technical specifications. It helps to show that you are in fact using an RF.
🙂