Fixing NSwag issue with duplicated enum values in the generated Swagger document, https://blog.dangl.me/archive/remove-duplicate-enum-entries-in-swagger-documents-with-nswag-in-aspnet-core/

HttpStatusCode.cs

public enum HttpStatusCode
{
    /* Excerpt from System.Net.HttpStatusCode */
    RedirectKeepVerb = 307,
    TemporaryRedirect = 307
}

NSwagPostProcess.cs

services.AddSwaggerDocument(c =>
{
    c.PostProcess = (x) =>
    {
        // Some enum classes use multiple integer values for the same value, e.g.
        // System.Net.HttpStatusCode uses these:
        // RedirectKeepVerb = 307
        // TemporaryRedirect = 307
        // MVC is configured to use the StringEnumConverter, and NJsonSchema errorenously
        // outputs the duplicates. For the example above, the value 'TemporaryRedirect' is
        // serialized twice, 'RedirectKeepVerb' is missing.
        // The following post process action should remove duplicated enums
        // See https://github.com/RSuter/NJsonSchema/issues/800 for more information
        foreach (var enumType in x.Definitions.Select(d => d.Value).Where(d => d.IsEnumeration))
        {
            var distinctValues = enumType.Enumeration.Distinct().ToList();
            enumType.Enumeration.Clear();
            foreach (var distinctValue in distinctValues)
            {
                enumType.Enumeration.Add(distinctValue);
            }
        }
    };
});