[wasm] Check for duplicate export names

test/mjsunit/wasm/export-table.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --expose-wasm
 
 load("test/mjsunit/wasm/wasm-constants.js");
 load("test/mjsunit/wasm/wasm-module-builder.js");
 
 (function testExportedMain() {
@@ skipping 54 matching lines @@
     ])
     .exportAs("0");
 
   var module = builder.instantiate();
 
   assertEquals("object", typeof module.exports);
   assertEquals("function", typeof module.exports["0"]);
 
   assertEquals(kReturnValue, module.exports["0"]());
 })();
+
+(function testExportNameClash() {
+  var builder = new WasmModuleBuilder();
+
+  builder.addFunction("one",   kSig_v_v).addBody([kExprNop]).exportAs("main");
+  builder.addFunction("two",   kSig_v_v).addBody([kExprNop]).exportAs("other");
+  builder.addFunction("three", kSig_v_v).addBody([kExprNop]).exportAs("main");
+
+  try {
+    builder.instantiate();
+    assertUnreachable("should have thrown an exception");
+  } catch (e) {
+    assertContains("Duplicate export", e.toString());
+  }
+})();