cgo: omit duplicate symbols in writeDefs

When the C API being used includes multiple names for the same
underlying symbol (e.g. multiple #define's for the same variable), then
cgo will generate the same placeholder variables for each name.  This
then prevents the code from compiling due to multiple declarations of
the same variable - so change cgo to only create one instance of the
variable for the underlying symbol.

R=rsc
CC=golang-dev
https://golang.org/cl/4826055

misc/cgo/test/Makefile

@@ -15,6 +15,7 @@ CGOFILES=\
 	issue1222.go\
 	issue1328.go\
 	issue1560.go\
+	duplicate_symbol.go\
 
 CGO_OFILES=\
 	callback_c.o\

misc/cgo/test/duplicate_symbol.go

@@ -0,0 +1,21 @@
+// Copyright 2010 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.
+
+// This file contains test cases for cgo.
+
+package cgotest
+
+/*
+int base_symbol = 0;
+
+#define alias_one base_symbol
+#define alias_two base_symbol
+*/
+import "C"
+
+import "fmt"
+
+func duplicateSymbols() {
+	fmt.Printf("%v %v %v\n", C.base_symbol, C.alias_one, C.alias_two)
+}

src/cmd/cgo/out.go

@@ -59,17 +59,21 @@ func (p *Package) writeDefs() {
 
 	fmt.Fprintf(fc, cProlog)
 
-	var cVars []string
+	cVars := make(map[string]bool)
 	for _, n := range p.Name {
 		if n.Kind != "var" {
 			continue
 		}
-		cVars = append(cVars, n.C)
 
+		if !cVars[n.C] {
 			fmt.Fprintf(fm, "extern char %s[];\n", n.C)
 			fmt.Fprintf(fm, "void *_cgohack_%s = %s;\n\n", n.C, n.C)
 
 			fmt.Fprintf(fc, "extern byte *%s;\n", n.C)
+
+			cVars[n.C] = true
+		}
+
 		fmt.Fprintf(fc, "void *·%s = &%s;\n", n.Mangle, n.C)
 		fmt.Fprintf(fc, "\n")