Ranking only one designer in all top positions will discourage other designers to join your contest and therefore you´ll have no new entries from new designers.
Something most Contest Holders don´t know is what´s the ranking for and how to rank entries.
How to Rank and Thank Designers: When it comes time for rankings, designers appreciate when you choose the top three CONCEPTS (as opposed to variations of the same design) for the gold, silver and bronze awards. If you want to rank two different concepts by the same designer in the top three, that's fine.
Also, rank as many entries as possible, because designers get points added to their overall site standing for each blue ribbon they get.
This is how points are distributed:
1st place - 50 points
2nd place - 20 points
3rd place - 10 points
4-6th place - 5 points
7-10 place - 3 points
11th+ (you can rank as many as you like) - 1 point
After that, each ranked entry gets one point for the designer. Nobody will be insulted by being ranked last, the points are still good! So if you pull some entries out of the "Not Interested" group and give them a rank, the designer gets some benefit from entering your contest.
Hope that's helpful, lots of contest holders don't realize this and most are glad to know. It's a good way to thank designers who don't win the contest prize, but who worked very hard to offer you lots of options to choose from for a great logo.
Thanks and Good Luck!