A Swagger `ISchemaFilter` that removes duplicate properties in `allOf` schemas

DuplicatePropertySchemaFilter.cs

    /// <summary>
    /// Represents a Swagger filter that removes duplicate properties in 'allOf'
    /// schema definitions.
    /// </summary>
    public class DuplicatePropertySchemaFilter : ISchemaFilter
    {
        private const string DefinitionsRefPrefix = "#/definitions/";

        /// <summary>
        /// Removes duplicate properties from the specified schema.
        /// </summary>
        /// <param name="schema">A schema to apply to the filter to.</param>
        /// <param name="context">
        /// A <see cref="SchemaFilterContext"/> that provides context for the
        /// filter operation.
        /// </param>
        public void Apply(Schema schema, SchemaFilterContext context)
        {
            if (schema.AllOf != null)
            {
                var knownProperties = new HashSet<string>();
                foreach (var subSchema in schema.AllOf.Select(x => ResolveSchema(x, context.SchemaRegistry)))
                {
                    subSchema.Properties = RemoveDuplicateProperties(subSchema, knownProperties);
                }
            }
        }

        private static Dictionary<string, Schema> RemoveDuplicateProperties(Schema schema, ISet<string> knownProperties)
        {
            var uniqueProperties = new Dictionary<string, Schema>(schema.Properties);

            foreach (var property in schema.Properties)
            {
                var isNew = knownProperties.Add(property.Key);
                if (!isNew)
                {
                    uniqueProperties.Remove(property.Key);
                }
            }

            return uniqueProperties;
        }

        private static Schema ResolveSchema(Schema schema, ISchemaRegistry registry)
        {
            if (string.IsNullOrEmpty(schema.Ref))
                return schema;

            if (!schema.Ref.StartsWith(DefinitionsRefPrefix))
                throw new ArgumentException($"The schema reference '{schema.Ref}' is not valid.", nameof(schema));

            var name = schema.Ref.Substring(DefinitionsRefPrefix.Length);
            return registry.Definitions[name];
        }
    }