@lightless - It is certainly a possibility that multiple creatives may submit the same name for instant domains, or to different contests at different times.
Although it is not very likely that if a name has been rejected in the marketplace, it will be approved the next time - however it is certainly a possibility. We continue to adjust our acceptance criteria based upon our understanding of the market dynamics. In those cases the recent entry which was submitted will likely be accepted. Unfortunately, given the volume of submissions we receive for Instant Domains, we do not have the ability to check whether the same submission was submitted in the past by a different creative.
However if you submit an entry in instant domain/ marketplace contest, only one creative will be able to submit it. The same entry can not be submitted again by a different creative in the same contest.