fix redundant WindowsDNS (#1456)

Co-authored-by: ForestL <45709305+forestl18@users.noreply.github.com>

dns/system_windows.go

@@ -8,6 +8,7 @@ import (
 	"syscall"
 	"unsafe"
 
+	"golang.org/x/exp/slices"
 	"golang.org/x/sys/windows"
 )
 
@@ -40,6 +41,9 @@ func dnsReadConfig() (servers []string, err error) {
 				// Unexpected type.
 				continue
 			}
+			if slices.Contains(servers, ip.String()) {
+				continue
+			}
 			servers = append(servers, ip.String())
 		}
 	}