I figured out that the block 19(the small one) is at the same position of the duplicated block and I removed it. Now it doesn't show any warning.
BUT IN THE FIND OBJECT OF DUPLICATE 64 IT WAS SHOWING THE GROUP OF ANOTHER OBJECT WHICH IS TOTALY UNRELATED WITH BLOCK 19.

After 'unduplication' of block 19, I AGAIN selected the Undo button from the edit menu to have that duplicated block back. Now when I render it says Duplicate 78 and 79. But this time when you search for the Item it won't point to the group and it works right. But then again I didn't save the file and reloaded the level. This time the same blocks warning, only the numbers are 19 and 70. But as I said I didn't touch ANYTHING. I loaded the file as it was before.

My problem's solved anyway.

Thank you guys.