Board Thread:Questions and Answers/@comment-31594898-20170326105723/@comment-93.16.135.125-20170326123646

For Grand Order, the gameplay is only the gameplay, it doesn't necessarily affect the story. The story is supposed to unfold with only Mashu and whatever Servant actually appears in the events, and I don't think there's any chapter where you summon two versions of the same person at the same time. That being said, the Grand Order summoning system is not the same as the Fuyuki Grail War. The Fuyuki system almost definitely has a lot more restrictions than the Grand Order system, so it shouldn't be a problem if Chaldea actually decided to summon both Caster and Lancer Cu at the same time, but for Fuyuki, that's very unlikely.

The Moon Cell cases of Bathory and Li are special. It's not something that can normally happen, but rules are made to be broken so that the story is interesting anyway. Mud corruption is also pretty unique. Kingu is not even a Servant, he's a living being when we meet him, just like Gilgamesh.