Duplicate __typename in response for nested union queries

[dkempner]

What happened?

When running a query with this shape, the response that's returned is technically not valid JSON because it has duplicate __typename properties.

Query

query {
  union {
    __typename
    ... on A {
      __typename
    }
    ... on B {
      __typename
    }
  }
}

Response

{
    "data": {
        "union": [
            {
                "__typename": "A",
                "__typename": "A"
            },
            {
                "__typename": "B",
                "__typename": "B"
            }
        ]
    }
}

What did you expect?

Duplicate properties should be deduplicated.

Minimal graphql.schema and models to reproduce

1. Clone https://github.com/dkempner/gqlgen-union-double-typename
2. go run server.go
3. Visit http://localhost:8080/
4. Open Chrome devtools to network tab
5. Execute the query saved in repro.grapqhl
6. Observe the "Response" tab has duplicate __typename properties

versions

• go run github.com/99designs/gqlgen version?
  • v0.17.56
• go version?
  • go version go1.23.3 darwin/arm64

[StevenACoffman]

Oh, wow. Yeah, that's definitely a bug. Thanks for reporting, and if you get a chance, I would love a PR to fix.