Please not another SittingDuck...
Well, not sure, but i guess there is no scoring system that can handle an unevenly distributed advantage of fighting against 8 instead of 9 opponents. The more 'ducks' the competition gets the more will the score spread and it will take ages to get a settled score. Its clearly a difference to fight 8 (or even less) instead of 9 opponents. Not to speak about the 'free' energy gain for the lucky bot who starts near the duck.
More ducks will reward prey-on-the-weak strategies more. Although this kind of strategy will always be rewarded to some extent in battles with 3 or more bots, with or without ducks.
As a note, there are intentional biases in robocode's score system that work to reward certain types of behavior anyway. Note the bullet damage and kill bonuses. Any way you do score will have some kind of biases or "value judgements" in it really. Even with the most pure "survivalist" scoring systems, a subjective judgement about what makes a "good bot" is being made.