Hi Biljana,
sorry to hear that you are experiencing this problem. I gave a look at what happens behind the scenes and found that apparently 38 items show the same problem. The list of item IDs is:
"7737190, 7737298, 7737456, 7737707, 7737780, 7737816, 7737822, 7737833, 7737973, 7738042, 7738071, 7738135, 7738149, 7738188, 7738360, 7738410, 7738416, 7738598, 7738926, 7738939, 7738980, 7739051, 7739134, 7739234, 7739235, 7739315, 7739367, 7739466, 7739602, 7739640, 7739793, 7739878, 7739922, 7740023, 7740043, 7740054, 7740440, 7740588" (they all belong to the WOK 7/30/2012 series of sources)
Can you confirm that the items you found belong to the list? (this way I'll know I am looking at the right problem).
As this is quite certainly a bug (and a very rare one), I will need to investigate in detail and find a way to make sure that it won't happen again. I will, of course, fix your own situation, but would like to investigate the issue first, if that's all right with you.
Could you please try to describe (with as much detail as possible) what you did while duplicate checking? I'm interested to know if you used the "reset" functions, if you manually added items to groups, if you manually created groups and if/when you have used the "mark automatically" routine(s).
I hope this information will be enough for me to find the cause all this. I'm very sorry for the trouble.
Many thanks for letting us know,
Sergio