fix multiple of same startup

Author: BrennanConroy

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

@@ -178,6 +178,7 @@ public IWebHostBuilder UseStartup([DynamicallyAccessedMembers(StartupLinkerOptio
             if (_builder.Properties.TryGetValue(_startupConfigName, out var startupObject) &&
                 object.ReferenceEquals(startupObject, startupType))
             {
+                _builder.Properties.Remove(_startupConfigName);
                 UseStartup(startupType, context, services);
             }
         });
@@ -205,6 +206,7 @@ void ConfigureStartup(HostBuilderContext context, IServiceCollection services)
             if (_builder.Properties.TryGetValue(_startupConfigName, out var startupObject) &&
                 object.ReferenceEquals(startupObject, startupFactory))
             {
+                _builder.Properties.Remove(_startupConfigName);
                 var webHostBuilderContext = GetWebHostBuilderContext(context);
                 var instance = startupFactory(webHostBuilderContext) ?? throw new InvalidOperationException("The specified factory returned null startup instance.");
                 UseStartup(instance.GetType(), context, services, instance);
@@ -325,6 +327,7 @@ public IWebHostBuilder Configure(Action<IApplicationBuilder> configure)
             if (_builder.Properties.TryGetValue(_startupConfigName, out var startupObject) &&
                 object.ReferenceEquals(startupObject, configure))
             {
+                _builder.Properties.Remove(_startupConfigName);
                 services.Configure<GenericWebHostServiceOptions>(options =>
                 {
                     options.ConfigureApplication = configure;
@@ -349,6 +352,7 @@ public IWebHostBuilder Configure(Action<WebHostBuilderContext, IApplicationBuild
             if (_builder.Properties.TryGetValue(_startupConfigName, out var startupObject) &&
                 object.ReferenceEquals(startupObject, configure))
             {
+                _builder.Properties.Remove(_startupConfigName);
                 services.Configure<GenericWebHostServiceOptions>(options =>
                 {
                     var webhostBuilderContext = GetWebHostBuilderContext(context);

src/Hosting/Hosting/test/GenericWebHostBuilderTests.cs

@@ -153,6 +153,29 @@ public async Task MultipleConfigureWebHostCallsWithUseStartupLastWins()
         await AssertResponseContains(server.RequestDelegate, "SecondStartup");
     }
 
+    [Fact]
+    public async Task MultipleConfigureWebHostCallsWithSameUseStartupOnlyRunsOne()
+    {
+        var server = new TestServer();
+
+        using var host = new HostBuilder()
+            .ConfigureWebHost(webHostBuilder =>
+            {
+                webHostBuilder
+                    .UseServer(server)
+                    .UseStartup<FirstStartup>();
+            })
+            .ConfigureWebHost(webHostBuilder =>
+            {
+                webHostBuilder
+                    .UseStartup<FirstStartup>();
+            })
+            .Build();
+
+        await host.StartAsync();
+        Assert.Single(host.Services.GetRequiredService<IEnumerable<FirstStartup>>());
+    }
+
     private async Task AssertResponseContains(RequestDelegate app, string expectedText)
     {
         var httpContext = new DefaultHttpContext();