bug 211: duplicate interface methods not caught by 6g

R=gri
DELTA=17  (17 added, 0 deleted, 0 changed)
OCL=35852
CL=35876

test/bugs/bug211.go

+// errchk $G $D/$F.go
+
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+type R interface { duplicate() }
+type S interface { duplicate() }
+type T interface { R; S }	// ERROR "duplicate"
+
+func main() {
+}

test/golden.out

@@ -170,3 +170,6 @@ BUG: bug196
 =========== bugs/bug210.go
 bugs/bug210.go:10: invalid recursive type []T
 BUG: should compile
+
+=========== bugs/bug211.go
+BUG: errchk: command succeeded unexpectedly