Fix AllProtocols with duplicates

src/SignalR/server/Core/src/Internal/DefaultHubProtocolResolver.cs

@@ -23,13 +23,12 @@ namespace Microsoft.AspNetCore.SignalR.Internal
             _logger = logger ?? NullLogger<DefaultHubProtocolResolver>.Instance;
             _availableProtocols = new Dictionary<string, IHubProtocol>(StringComparer.OrdinalIgnoreCase);
 
-            // We might get duplicates in _hubProtocols, but we're going to check it and overwrite in just a sec.
+            foreach (var protocol in availableProtocols)
-            _hubProtocols = availableProtocols.ToList();
-            foreach (var protocol in _hubProtocols)
             {
                 Log.RegisteredSignalRProtocol(_logger, protocol.Name, protocol.GetType());
                 _availableProtocols[protocol.Name] = protocol;
             }
+            _hubProtocols = _availableProtocols.Values.ToList();
         }
 
         public virtual IHubProtocol GetProtocol(string protocolName, IReadOnlyList<string> supportedProtocols)

src/SignalR/server/SignalR/test/Internal/DefaultHubProtocolResolverTests.cs

@@ -84,6 +84,22 @@ namespace Microsoft.AspNetCore.SignalR.Common.Protocol.Tests
             Assert.Same(jsonProtocol2, resolvedProtocol);
         }
 
+        [Fact]
+        public void AllProtocolsOnlyReturnsLatestOfSameType()
+        {
+            var jsonProtocol1 = new NewtonsoftJsonHubProtocol();
+            var jsonProtocol2 = new NewtonsoftJsonHubProtocol();
+            var resolver = new DefaultHubProtocolResolver(new[] {
+                jsonProtocol1,
+                jsonProtocol2
+            }, NullLogger<DefaultHubProtocolResolver>.Instance);
+
+            var hubProtocols = resolver.AllProtocols;
+            Assert.Equal(1, hubProtocols.Count);
+
+            Assert.Same(jsonProtocol2, hubProtocols[0]);
+        }
+
         public static IEnumerable<object[]> HubProtocolNames => HubProtocolHelpers.AllProtocols.Select(p => new object[] {p.Name});
     }
 }