added test for #337

Author: maximv

test/DryIoc.IssuesTests/GHIssue337_Singleton_is_created_twice.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using NUnit.Framework;
+
+namespace DryIoc.IssuesTests
+{
+    [TestFixture]
+    public class GHIssue337_Singleton_is_created_twice
+    {
+        [Test]
+        public void Test()
+        {
+            var container = new Container();
+            container.Register<A>(Reuse.Singleton);
+
+            var ts = new Task[8];
+            for (int i = 0; i < ts.Length; i++)
+            {
+                ts[i] = Task.Run(() => container.Resolve<A>());
+            }
+
+            Task.WaitAll(ts);
+        }
+
+        class A 
+        { 
+            public static int Counter;
+            public A() 
+            {
+                Thread.Sleep(100);
+                Console.WriteLine(++Counter);
+            }
+        }
+
+    }
+}