Quoting blue post here: " You won't find a second copy of a common object until you have found all the artifacts of that race. If you get stuck at a certain skill level of archaeology or character level, you may find an artifact more than once until you reach the next tier."
"Some artifacts are rare. These always make a blue or purple item." "You will never get a rare artifact more than once."
Source: http://www.mmo-champion.com/content/...ogy-Blue-Posts
It cleary says you wont find a second copy of a common artifact unti you find all artifacts for that race. Notice it says all artifacts, not all common artifacts. I am at 525 and i have got like 4 repeated Night elf grays and only two of the NE rares. Is the blue post wrong? Or is archaeology bugged? or is there some mechanic we are not aware of? for example i have digged very few dwarf items so i got no dwarf rares.