That's my experience:
I've read and tried every piece of advice here in the forum: the only solution that worked for me was to format the SD card (a level "4" Kingston 16 GB) and put the songs directly in the
music folder of the sd not by phone usb-connected but putting first the SDinside my laptop.
But then i wasn't happy and i've stumbled on a post in microsoft answers:
Why is my music being duplicated on my Windows 8 phone? - Microsoft Community
And that's was the procedure i've followed (by Jay_answers):
"1. Take out your SD card from the phone. Format it. don't put it back to the phone yet.
2. Add a couple of songs to your HTC 8S
(Lumia 820, in my case) . Since your SD card is no longer in the phone, the music will be stored directly on the phone. Since 8S (
Lumia 820)has only 4GB memory, suggest you only add a couple of tracks.
3. Check the phone. Do you still see duplicates? If you still see duplicates, it suggests that something is wrong with the music app on the phone. There is nothing specific we can do about the music app, so phone reset is the only option.
4. In step 3, if you don't see duplicates, then we have realized that the issues lies with the SD card storage and not with the phone itself.
5. Add a couple of tracks to the SD card (which has been formatted in step 1), and give it a go with the phone. If you see duplicates, then it confirms a issue with the SD. Get another SD card.
Hope it helps. Let me know how it goes, as this is a new issue
Good luck
"
So i've tired the first steps and i've noticed that, copying the music via usb directly to my phone, produced the infamous "songs duplication"... So it wasn't the sd card but the phone itself that had the problems but, following the various suggestions in the previous posts, i've restored my phone several times during the past days.........
And then i've realized that my phone had another firmware (a "country variant") beacuse i hate when i switch on the phone seeing the operator logo. So here's the fact: i've used the "nokia care" to refurbish my phone with an original firmware put the card back on, downloaded the desktop application of Microsoft, connected my phone to usb and copyed all my music: no duplication at all!
Notice that in the previous weeks i've soffered that nightmares of 3-4 and even 7 times the same song listed.....
Obviously every mp3 is tagged using mp3tag 2.4 or MusicBrainz Picard: at least now my collection has a perfect tagging system!
That said: xbox-music is a mess, the metro application for wp8 is a mess, the desktop app is an "app preview" but at least works but the real problem - for me - was the non-original firmware on the phone.
I wish to thank everyone in the forum that has given preciuos advices, with their posts, during these weeks.
Sincerelly
Daisho2000