[@VitaVitaAR][@Rin] Oh! There were two claims on Rider? Yikes, that's quite a situation to have on your hands. 

But it's ok. Accidents happen. Though i guess i can try to throw in a couple ideas to attempt to help the situation. :)

1. There could be a "false" servant in the Grail War who is in the container of one of the seven servant classes, similar to the situation from Fate/Stay Night, who was summoned in an erroneous way (like Medea, a Caster servant, having summoned Kojiro who was a fake servant) or the like that allowed them to be summoned into the Grail War. This could be while a "true" version of the servant was also summoned.

2. Maybe one of the extra "irregular" classes, like the Shielder class from Fate/GO or the Avenger class like the Einzberns tried to summon in the Third Holy Grail War, was summoned due to some kind of interference by one of three families who made the original Grail War (Einzberns, Matou, or Tohsaka trying to get in on the action, perhaps via a proxy or Kiritsugu-like figure) or some other error in the system or an attempted summoning.