Like I said in the title, I think identical twins would be a good addition to the game. Additionally, it requires no art to be drawn, so it wouldn't be too much work to implement. As for how it would work, I think I have a good idea.
I think it shouldn't be a proper mutation, as irl identical twins isn't a genetic trait. I think it would just be a random event that can happen sometimes. It would be rolled before any of the cubs, and if it succeeds, then instead of generating each cub individually, it just rolls for one cub, and applies those genetics to all of the cubs. Since it's technically not a mutation, I don't think it should be affected by low fertility or other mutation modifiers.