That there is a common effect for similar actions is beyond dispute, the teachings of the Buddha on kamma show this very clearly, and that karmic effect is modified by intention, the object that is acted upon (killing an arhat as opposed to killing a mosquito), the quantity and quality of the action and satisfaction gained from an action is also indisputable.
But to say that the karma vipakka will be indentical is going a bit too far.
Okay, the Abhidha...ma texts basically outline the limited range of sentient experience, so it can be said that since we are dealing with such a limited range of experience then the combinations that can occur are also limited and that there is a distinct possibility that the karma vipakka for a group of individuals engaged in the same action may be similar. But I think the term "similar" is of importance here. The abovementioned passages seem to say that the karma vipakka will be the same. This, I feel that this is too far fetched. Why? Because sentient beings have been cycling through samsara forever, so the possibility that their individual mind streams will bring about an identical outcome is kinnda difficult to believe.
If one looks to the Abhidha...ma "lists" one finds a limited but still seemingly vast array of experiences, if one combines these with the structures of dependency outlined in the Patthanuddesa Dipani http://www.dharmaweb.org/index.php/The_ ... _Relations
then the possible number combinations shoot right off the end of the scale!
Though I don't find the theory of group karma improbable the lack of references to it in Sutta and Sutra leads me to believe that it is a later development in Buddhist theory which, of course, is not a negative trait but is a point of concern. Maybe the theory requires better elucidation/analysis based on realisations?