Fixed de-duping hosting startup assemblies

src/Hosting/Hosting/src/GenericHost/GenericWebHostBuilder.cs

@@ -129,15 +129,23 @@ private void ExecuteHostingStartups()
             }
 
             var exceptions = new List<Exception>();
+            var processed = new HashSet<Assembly>();
+
             _hostingStartupWebHostBuilder = new HostingStartupWebHostBuilder(this);
 
             // Execute the hosting startup assemblies
-            foreach (var assemblyName in webHostOptions.GetFinalHostingStartupAssemblies().Distinct(StringComparer.OrdinalIgnoreCase))
+            foreach (var assemblyName in webHostOptions.GetFinalHostingStartupAssemblies())
             {
                 try
                 {
                     var assembly = Assembly.Load(new AssemblyName(assemblyName));
 
+                    if (!processed.Add(assembly))
+                    {
+                        // Already processed, skip it
+                        continue;
+                    }
+
                     foreach (var attribute in assembly.GetCustomAttributes<HostingStartupAttribute>())
                     {
                         var hostingStartup = (IHostingStartup)Activator.CreateInstance(attribute.HostingStartupType)!;

src/Hosting/Hosting/src/WebHostBuilder.cs

@@ -229,14 +229,21 @@ private IServiceCollection BuildCommonServices(out AggregateException? hostingSt
             if (!_options.PreventHostingStartup)
             {
                 var exceptions = new List<Exception>();
+                var processed = new HashSet<Assembly>();
 
                 // Execute the hosting startup assemblies
-                foreach (var assemblyName in _options.GetFinalHostingStartupAssemblies().Distinct(StringComparer.OrdinalIgnoreCase))
+                foreach (var assemblyName in _options.GetFinalHostingStartupAssemblies())
                 {
                     try
                     {
                         var assembly = Assembly.Load(new AssemblyName(assemblyName));
 
+                        if (!processed.Add(assembly))
+                        {
+                            // Already processed, skip it
+                            continue;
+                        }
+
                         foreach (var attribute in assembly.GetCustomAttributes<HostingStartupAttribute>())
                         {
                             var hostingStartup = (IHostingStartup)Activator.CreateInstance(attribute.HostingStartupType)!;

src/Hosting/Hosting/test/WebHostBuilderTests.cs

@@ -903,6 +903,27 @@ public void Build_RunsHostingStartupAssembliesIfSpecified(IWebHostBuilder builde
             using (var host = builder.Build())
             {
                 Assert.Equal("1", builder.GetSetting("testhostingstartup1"));
+                Assert.Equal("1", builder.GetSetting("testhostingstartup1_calls"));
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(DefaultWebHostBuildersWithConfig))]
+        public void Build_RunsDeduplicatedHostingStartupAssembliesIfSpecified(IWebHostBuilder builder)
+        {
+            var fullName = typeof(TestStartupAssembly1.TestHostingStartup1).Assembly.FullName;
+            var name = typeof(TestStartupAssembly1.TestHostingStartup1).Assembly.GetName().Name;
+
+            builder = builder
+                .CaptureStartupErrors(false)
+                .UseSetting(WebHostDefaults.HostingStartupAssembliesKey, fullName + ";" + name)
+                .Configure(app => { })
+                .UseServer(new TestServer());
+
+            using (var host = builder.Build())
+            {
+                Assert.Equal("1", builder.GetSetting("testhostingstartup1"));
+                Assert.Equal("1", builder.GetSetting("testhostingstartup1_calls"));
             }
         }
 

src/Hosting/test/testassets/TestStartupAssembly1/TestHostingStartup1.cs

@@ -1,6 +1,7 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System.Globalization;
 using Microsoft.AspNetCore.Hosting;
 
 [assembly: HostingStartup(typeof(TestStartupAssembly1.TestHostingStartup1))]
@@ -11,8 +12,17 @@ public class TestHostingStartup1 : IHostingStartup
     {
         public void Configure(IWebHostBuilder builder)
         {
+            var calls = builder.GetSetting("testhostingstartup1_calls");
+            var numCalls = 1;
+
+            if (calls != null)
+            {
+                numCalls = int.Parse(calls, CultureInfo.InvariantCulture) + 1;
+            }
+
             builder.UseSetting("testhostingstartup1", "1");
             builder.UseSetting("testhostingstartup_chain", builder.GetSetting("testhostingstartup_chain") + "1");
+            builder.UseSetting("testhostingstartup1_calls", numCalls.ToString(CultureInfo.InvariantCulture));
         }
     }
 }