Detect duplicate startups in HostingStartupAssemblies (#1183)

src/Microsoft.AspNetCore.Hosting/WebHostBuilder.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
+using System.Linq;
 using System.Reflection;
 using System.Runtime.ExceptionServices;
 using Microsoft.AspNetCore.Hosting.Builder;
@@ -195,7 +196,7 @@ namespace Microsoft.AspNetCore.Hosting
                 var exceptions = new List<Exception>();
 
                 // Execute the hosting startup assemblies
-                foreach (var assemblyName in _options.HostingStartupAssemblies)
+                foreach (var assemblyName in _options.HostingStartupAssemblies.Distinct(StringComparer.OrdinalIgnoreCase))
                 {
                     try
                     {

test/Microsoft.AspNetCore.Hosting.Tests/ConfigureBuilderTests.cs

@@ -9,7 +9,7 @@ using Microsoft.AspNetCore.Hosting.Internal;
 using Microsoft.Extensions.DependencyInjection;
 using Xunit;
 
-namespace Microsoft.AspNetCore.Hosting.Tests 
+namespace Microsoft.AspNetCore.Hosting.Tests
 {
     public class ConfigureBuilderTests
     {

test/Microsoft.AspNetCore.Hosting.Tests/WebHostBuilderTests.cs

@@ -234,6 +234,36 @@ namespace Microsoft.AspNetCore.Hosting
             }
         }
 
+        [Fact]
+        public async Task MultipleStartupAssembliesSpecifiedOnlyAddAssemblyOnce()
+        {
+            var provider = new TestLoggerProvider();
+            var assemblyName = "RandomName";
+            var data = new Dictionary<string, string>
+            {
+                { WebHostDefaults.ApplicationKey,  assemblyName },
+                { WebHostDefaults.HostingStartupAssembliesKey, assemblyName }
+            };
+            var config = new ConfigurationBuilder().AddInMemoryCollection(data).Build();
+
+            var builder = CreateWebHostBuilder()
+                 .UseConfiguration(config)
+                 .ConfigureLogging((_, factory) =>
+                 {
+                     factory.AddProvider(provider);
+                 })
+                .UseServer(new TestServer());
+
+            // Verify that there was only one exception throw rather than two.
+            using (var host = (WebHost)builder.Build())
+            {
+                await host.StartAsync();
+                var context = provider.Sink.Writes.Where(s => s.EventId.Id == LoggerEventIds.HostingStartupAssemblyException);
+                Assert.NotNull(context);
+                Assert.Single(context);
+            }
+        }
+
         [Fact]
         public void HostingContextContainsAppConfigurationDuringConfigureLogging()
         {