graphql-dotnet: Error about duplicate union type.

Summary

I sometimes stumble upon a strange error in our GraphQL schema related to union types. I did my best to search for answer but haven’t find anyone with a similar issue so far, which warrants me to create this issue.

Relevant information

The error is related to union types and looks as following:

{
  "error": "Invalid schema Error:\nUnion type ProductDrawsResult can only include type UpstreamServiceFailed once."
}

I’ve seen this happen before for other union types in our schema, so it does not seem related to this specific type.

I’ll post the code of this type nonetheless for you guys to take a look:

internal sealed class ProductDrawsResultType : UnionGraphType
    {
        public ProductDrawsResultType()
        {
            Name = "ProductDrawsResult";

            Type<ProductDrawsType>();
            Type<UpstreamServiceFailedType>();
        }
    }

The query still works as expected, but I can’t load the schema.

It puzzles me, does anyone have a clue?

Environment (if relevant)

GraphQL 4.7.1, OS Linux, .NET 6.0