Q: All songs in itunes library dupliated

if all came from iTunes try delete and re-download

How to find and remove duplicate items in your iTunes library

http://support.apple.com/kb/ht2905

Downloading past purchases from the iTunes Store, App Store, and iBooks Store

http://support.apple.com/kb/ht2519

How to delete content you've downloaded from the iTunes Store, App Store, iBooks Store, or Mac App Store

http://support.apple.com/kb/ht3842

iTunes: Finding lost media downloads

http://support.apple.com/kb/TS1408

good luck

Apple's official advice on duplicates is here... HT2905: How to find and remove duplicate items in your iTunes library. It is a manual process and the article fails to explain some of the potential pitfalls.

Use Shift > View > Show Exact Duplicate Items to display duplicates as this is normally a more useful selection. You need to manually select all but one of each group to remove. Sorting the list by Date Added may make it easier to select the appropriate tracks, however this works best when performed immediately after the dupes have been created.  If you have multiple entries in iTunes connected to the same file on the hard drive then don't send to the recycle bin.

Use my DeDuper script if you're not sure, don't want to do it by hand, or want to preserve ratings, play counts and playlist membership. See this thread for background and please take note of the warning to backup your library before deduping.

(If you don't see the menu bar press ALT to show it temporarily or CTRL+B to keep it displayed)

Alternativey see this migrate iTunes library post, you may want to revisit the transfer method.

tt2