Prevent synthetic upstreams without addresses from failing duplicate ip/port validation

agent/structs/structs.go

@@ -1189,7 +1189,9 @@ func (s *NodeService) Validate() error {
 			}
 			addr = net.JoinHostPort(addr, fmt.Sprintf("%d", u.LocalBindPort))
 
-			if _, ok := bindAddrs[addr]; ok {
+			// Centrally configured upstreams will fail this check if there are multiple because they do not have an address/port.
+			// Only consider non-centrally configured upstreams in this check since those are the ones we create listeners for.
+			if _, ok := bindAddrs[addr]; ok && !u.CentrallyConfigured {
 				result = multierror.Append(result, fmt.Errorf(
 					"upstreams cannot contain duplicates by local bind address and port; %q is specified twice", addr))
 				continue

agent/structs/structs_test.go

@@ -768,6 +768,24 @@ func TestStructs_NodeService_ValidateConnectProxy(t *testing.T) {
 			},
 			"upstreams cannot contain duplicates",
 		},
+		{
+			"connect-proxy: Centrally configured upstreams can have duplicate ip/port",
+			func(x *NodeService) {
+				x.Proxy.Upstreams = Upstreams{
+					{
+						DestinationType:     UpstreamDestTypeService,
+						DestinationName:     "foo",
+						CentrallyConfigured: true,
+					},
+					{
+						DestinationType:     UpstreamDestTypeService,
+						DestinationName:     "bar",
+						CentrallyConfigured: true,
+					},
+				}
+			},
+			"",
+		},
 		{
 			"connect-proxy: Upstreams duplicated by ip and port",
 			func(x *NodeService) {