In the Fate universe, is there a reason why the Servants summoned in each Holy Grail War belong to different classes?
Using relics and additional incantations during the summoning ritual, Masters have some degree of control over the Class (e.g. Matou Kariya's additional incantations to choose the Berserker Class) and exact identity of the Servant.
What would happen if two Masters both tried the same Class using an incantation (e.g., Berserker), or even used (part of) the same relic to summon the same Servant? For example:
What would happen if Waver Velvet had only stolen part of Rider's (Iskandar) cloak, unbeknownst to Kayneth Archibald, and both Masters tried to summon Rider?