a lot of songs is doubled in my collection. For example, on collection sorted by artist i got :

-Atomica
-Audio two
-a Perfect Circle
-a Perfect Circle

they got exactly same songs: first Perfect Circle got /media/disk/Muzyka/A Perfect Circle/Thirteenth Step/01 The Package.mp3
and second perfect circle got /media/disk/Muzyka/A Perfect Circle/Thirteenth Step/01 The Package.mp3

I mean, it's not tagging issue, but some songs get double (but not all, just about 30% of my collection).

Why that's happends, and how to fix it? updating collection didn't help.

But, what's interesting, if i type "Perfect Circle" in search window, it returns only one result (it isnt doubled any more).

Without knowing which version of Amarok you're using, here are some things that can fix duplicates where there isn't actually a duplicate file...

Instead of updating, try a full rebuild of the collection and see if the problem goes away.
Try changing the "Group By..." setting, then change it back again.
Check your music directory for symlinks.