StructureMapIssue.cs

using System;
using StructureMap;

namespace ConsoleApplication5
{
    static class Program
    {
        public interface ISayHello
        {
            void SayHello();
        }

        public class SayHelloImpl : ISayHello
        {
            private readonly Guid _id = Guid.NewGuid();
            public void SayHello() => Console.WriteLine($"Hello {_id}");
        }

        private static IContainer GetContainer()
        {
            return new Container(c =>
            {
                c.For<ISayHello>().Use<SayHelloImpl>();
                c.For<ISayHello>().Add<SayHelloImpl>().Named("A");
            });
        }

        public static void Main()
        {
            using (var container = GetContainer())
            {
                var a = container.GetInstance<Func<ISayHello>>();
                a().SayHello(); // ID 1
                a().SayHello(); // ID 2

                // Via the factory it reuses the same instance
                var a1 = container.GetInstance<Func<string, ISayHello>>();
                a1("A").SayHello(); // ID 3
                a1("A").SayHello(); // ID 3 (I would expect another one)

                var a2 = container.GetInstance<Func<string, ISayHello>>();
                a2("A").SayHello(); // ID 4
                a2("A").SayHello(); // ID 4 (I would expect another one)

                // Via explicit GetInstance calls it returns unique objects
                container.GetInstance<ISayHello>("A").SayHello();   // ID 5
                container.GetInstance<ISayHello>("A").SayHello();   // ID 6

                Console.ReadLine();
            }
        }
    }
}