<p>1/ It's very unlikely because it's a paradox, but it's probably not impossible. The Third War had two aspects of the same Saber summoned together as 2 Servants by the Edelfelt sisters<span style="font-weight:normal;">, they were also both Sabers</span> so that's a bit of a special case. </p><p>2/ Different aspects of the same person, different time in their life, slightly different personality from the difference in experience: although they'll both remember their entire life, they were probably in a different mindset in the age they were summoned as. I don't get what you mean by "totally different beings." They're two guys standing next to each other who happen to have the same mind and memories. Kid Alexander and kid Gilgamesh in Grand Order have their memories from being an adult but have a cheerful personality as a kid. </p><p>3/ The Caster Gilgamesh in Babylon is the one that's still alive, he sealed his treasury and stopped adventuring to rule his country, but he still uses magic from a distance, so he's a Caster. Then he dies and his spirit lingers in the Underworld, and he has access to his treasury (and Ea) so he becomes classified as an Archer. </p>

<p>Also, in a normal Fuyuki Grail War with the Servant system, no, a Servant can't change their class just by switching weapon. Gilgamesh doesn't become Saber, Lancer or Caster by grabbing hold of a different sword, spear or staff from his treasury. The Servant vessel is summoned once and that's it, that's the Servant system. </p>

<p>Ah, thank you so much. This helped very much. </p>

<p>1. This has occurred in the Grand Orders with more than 1 of an individual popping up and in gameplay, you of course can stack a party with multiple versions of the same person. </p><p>2. As the above user said, no, they're the same person, but as its described I believe, in the original FSN, though don't quote me on that, it's like ripping two pages out of a book. They're the same book still, but what's on the pages is different. </p><p>3. Several servants have been shown to 'change class' due to certain circumstances being met. This is mostly seen in the Moon Cell wars, where both Li Shuwen and Elizabeth Bathory are summoned as different classes, but both end up becoming Berserkers. Strange Fake has True Archer, whom, upon being exposed to Fuyuki Grail mud is also of the Avenger class. Grand Order also has several examples of enemies changing classes throughout, such as Kingu in Babylon and Hessian Lobo in Shinjuku. </p>

<p>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. </p><p>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. </p>

<p>I see. It's been a few years since I've watched a playthrough of stay night, so I guess it would be a good time to revisit it to see if it does mention it. </p>