I'm seeing duplicated images and videos on Gallery
That's not duplicates. If you are the "device owner", /storage/emulated/0/ is your storage area (where your files etc. reside). /storage/emulated/legacy/ is a symbolic link, always pointing to the logged-in user's storage. That is, if you are the device owner, and you are logged in, both locations are identical. If some apps show the contained files as "dupes", they obviously have problems interpreting symlinks.

While for the gallery, this sounds pretty strange (if we are talking about the stock gallery, and not a 3rd party app): the very same Android developers who designed the multi-user capabilities should have been aware of that. Still, this is not the first time I heard about it. I cannot find the other issue currently, but a solution is as follows:

Go to Settings&rarr;Apps&rarr;Manage Apps, and scroll to the Gallery app. Open its entry, tap "Clear Cache" and "Delete Data". Tap the back key, and repeat the same for the "Media Storage" app.

Don't be afraid about losing data: the Media Scanner will re-collect them at its next run. You can trigger the scan by either...


powering off/on the device (bah!)
going to Settings&rarr;Storage and safely remove the SDCard, then mount it again (OK for one-time use :)
using a helper app like e.g. SDrescan or Rescan SD Card! (if you need it more often)


Once the Media Scanner is finished, all your media should re-appear in the Gallery app. Hopefully without the "dupes" :)