Hi! When you say "no credit," do you mean that you didn't get a good item drop from Diablo, or that you weren't given credit for the quest (and therefore weren't able to move on to the next difficulty level)?
If you're talking about item drops, then you're absolutely right, D only drops a special item if the person who lands the killing blow hasn't killed him before.
As far as quest credit, that depends on who created the game. Before killing D, check your quest screen and make sure that the quest is available. If it's not, you'll never get credit for killing D in that game.
