FilterComparer.cs

 private class FilterComparer : IComparer<Filter>
 {
            public int Compare(Filter x, Filter y)
            {
                // Nulls always have to be less than non-nulls
                if (x == null && y == null)
                {
                    return 0;
                }
                if (x == null)
                {
                    return -1;
                }
                if (y == null)
                {
                    return 1;
                }

                // Sort first by order...

                if (x.Order < y.Order)
                {
                    return -1;
                }
                if (x.Order > y.Order)
                {
                    return 1;
                }

                // ...then by scope

                if (x.Scope < y.Scope)
                {
                    return -1;
                }
                if (x.Scope > y.Scope)
                {
                    return 1;
                }

                return 0;
            }
        }
    }

RemoveDuplicates.cs

private IEnumerable<Filter> RemoveDuplicates(IEnumerable<Filter> filters)
        {
            HashSet<Type> visitedTypes = new HashSet<Type>();

            foreach (Filter filter in filters)
            {
                object filterInstance = filter.Instance;
                Type filterInstanceType = filterInstance.GetType();

                if (!visitedTypes.Contains(filterInstanceType) || AllowMultiple(filterInstance))
                {
                    yield return filter;
                    visitedTypes.Add(filterInstanceType);
                }
            }
        }