When initialising the same schema for multiple ServiceCollection's you can only do this when calling UseGraphQL before you run AddGraphQL on the second collection.

var service1Collection = new ServiceCollection();
var service2Collection = new ServiceCollection();

service1Collection.AddGraphQL<CandleSchema>(options =>
{
    options.AddGraphType<Candle>();
});
service2Collection.AddGraphQL<CandleSchema>(options =>
{
    options.AddGraphType<Candle>();
});

This produces the error: The schema type CandleSchema has already been registered. Each schema type may only be registered once with GraphQL.