Multiple response types in ASP.NET Core with Swagger, see https://blog.dangl.me/archive/different-response-schemas-in-aspnet-core-swagger-api-definition-with-nswag/

Controller_Fixed.cs

[HttpGet("")]
[LightQuery]
[Authorize(Policy = DanglIdentityConstants.Authorization.AUTHENTICATION_CONNECTOR_OR_ADMIN_POLICY_NAME)]
[NSwag.Annotations.SwaggerResponse((int)HttpStatusCode.OK, typeof(PaginationResult<ClientGet>))]
[NSwag.Annotations.SwaggerResponse((int)HttpStatusCode.OK, typeof(IEnumerable<ClientGet>))]
public IActionResult GetAllClients(string filter = null)
{
    var clientsQuery = _identityServerClientsRepository.GetAllClients()
        .ProjectTo<ClientGet>();
    if (!string.IsNullOrWhiteSpace(filter))
    {
        clientsQuery = clientsQuery.Where(c => c.ClientName.Contains(filter) || c.ClientUri.Contains(filter));
    }
    return Ok(clientsQuery);
}

Controller_Invalid.cs

[HttpGet("")]
[LightQuery]
[Authorize(Policy = DanglIdentityConstants.Authorization.AUTHENTICATION_CONNECTOR_OR_ADMIN_POLICY_NAME)]
[ProducesResponseType(typeof(PaginationResult<ClientGet>), 200)]
[ProducesResponseType(typeof(ClientGet), 200)]
public IActionResult GetAllClients(string filter = null)
{
    var clientsQuery = _identityServerClientsRepository.GetAllClients()
        .ProjectTo<ClientGet>();
    if (!string.IsNullOrWhiteSpace(filter))
    {
        clientsQuery = clientsQuery.Where(c => c.ClientName.Contains(filter) || c.ClientUri.Contains(filter));
    }
    return Ok(clientsQuery);
}

swagger_fixed.json

{
  "operationId": "Clients_GetAllClients",
  "responses": {
    "200": {
      "x-nullable": true,
      "description": "",
      "schema": {},
      "x-expectedSchemas": [
        {
          "description": "",
          "schema": {
            "$ref": "#/definitions/PaginationResultOfClientGet"
          }
        },
        {
          "description": "",
          "schema": {
            "type": "array",
            "items": {
              "$ref": "#/definitions/ClientGet"
            }
          }
        }
      ]
    }
  }
}

swagger_invalid.json

{
  "operationId": "Clients_GetAllClients",
  "responses": {
    "200": {
      "x-nullable": true,
      "description": "",
      "schema": {
        "$ref": "#/definitions/PaginationResultOfClientGet"
      }
    }
  }
}

SwaggerGenerated.cs

// With the framework-provided ProducesResponseTypeAttribute, it just used the first response type 
System.Threading.Tasks.Task<PaginationResult<ClientGet>> GetAllClientsAsync(string filter);

// While the NSwag specific attribute correctly recognized there are two schemas and thus typed the response as 'object'
System.Threading.Tasks.Task<object> GetAllClientsAsync(string filter);

UnitTests.cs

[Fact]
public async Task GeneratesPaginationResultAndEnumerableForGetAllClientsAction()
{
    _url = "http://example.com/swagger/swagger.json";
    await MakeRequest();
    var responseString = await _response.Content.ReadAsStringAsync();
    var json = JObject.Parse(responseString);
    var expectedSchemas = json["paths"]["/api/clients"]["get"]["responses"]["200"]["x-expectedSchemas"] as JArray;

    Assert.NotNull(expectedSchemas);
    Assert.Equal(2, expectedSchemas.Count);
}

[Fact]
public void NoControllerHasMultipleResponseTypesDefined()
{
    // There was a behavior change between NSwag 11.19 and 11.20. Previously,
    // having two attributes of type [ResponseType] was OK and resulted in
    // both being considered int he Swagger specification. After the update,
    // only the first attribute was considered, thus creating incorrect
    // Swagger clients.

    var controllerMethods = typeof(Dangl.Identity.Startup)
        .GetTypeInfo()
        .Assembly
        .DefinedTypes
        .Where(t => typeof(Microsoft.AspNetCore.Mvc.ControllerBase).GetTypeInfo().IsAssignableFrom(t))
        .SelectMany(controllerType => controllerType.DeclaredMethods)
        .ToList();

    Assert.NotEmpty(controllerMethods);

    foreach (var controllerMethod in controllerMethods)
    {
        var methodAttributes = controllerMethod.CustomAttributes
            .Where(att => att.AttributeType == typeof(Microsoft.AspNetCore.Mvc.ProducesResponseTypeAttribute))
            .Where(att => att.ConstructorArguments.Count == 2) // Only taking the overload which defines a status code and a response type
            .Where(att => att.ConstructorArguments.Any(arg => arg.Value is int statusCode && (statusCode >= 200 && statusCode <= 299))) // Only where success status codes are defined
            .ToList();
        Assert.False(methodAttributes.Count > 1);
    }
}