diff --git a/dev/core/src/com/google/gwt/dev/jjs/impl/GenerateJavaScriptAST.java b/dev/core/src/com/google/gwt/dev/jjs/impl/GenerateJavaScriptAST.java
index 287f1c1..8cfa63e 100644
--- a/dev/core/src/com/google/gwt/dev/jjs/impl/GenerateJavaScriptAST.java
+++ b/dev/core/src/com/google/gwt/dev/jjs/impl/GenerateJavaScriptAST.java
@@ -153,6 +153,7 @@
 import com.google.gwt.dev.util.collect.IdentityHashSet;
 import com.google.gwt.dev.util.collect.Lists;
 import com.google.gwt.dev.util.collect.Maps;
+import com.google.gwt.dev.util.collect.Sets;
 
 import java.io.StringReader;
 import java.util.ArrayList;
@@ -774,6 +775,9 @@
           }
         }
       }
+      
+      // TODO(zundel): Check that each unique method has a unique
+      // name / poly name.
     }
 
     @Override
@@ -1419,7 +1423,13 @@
       if (x.getSuperClass() != null && !alreadyRan.contains(x)) {
         accept(x.getSuperClass());
       }
+      
+      return super.visit(x, ctx);
+    }
 
+    @Override
+    public boolean visit(JDeclaredType x, Context ctx) {
+      checkForDupMethods(x);
       return true;
     }
 
@@ -1610,6 +1620,20 @@
       return false;
     }
 
+    private void checkForDupMethods(JDeclaredType x) {
+      // Sanity check to see that all methods are uniquely named.
+      List<JMethod> methods = x.getMethods();
+      Set<String> methodSignatures = Sets.create();
+      for (JMethod method : methods) {
+        String sig = method.getSignature();
+        if (methodSignatures.contains(sig)) {
+          throw new InternalCompilerException("Signature collision in Type " + x.getName()
+              + " for method " + sig);
+        }
+        methodSignatures = Sets.add(methodSignatures, sig);
+      }
+    }
+
     private JsExpression createAssignment(JsExpression lhs, JsExpression rhs) {
       return new JsBinaryOperation(lhs.getSourceInfo(), JsBinaryOperator.ASG, lhs, rhs);
     }
diff --git a/dev/core/src/com/google/gwt/dev/jjs/impl/JsoDevirtualizer.java b/dev/core/src/com/google/gwt/dev/jjs/impl/JsoDevirtualizer.java
index 75b2522..31d952b 100644
--- a/dev/core/src/com/google/gwt/dev/jjs/impl/JsoDevirtualizer.java
+++ b/dev/core/src/com/google/gwt/dev/jjs/impl/JsoDevirtualizer.java
@@ -151,6 +151,12 @@
   private final JMethod isJavaObjectMethod;
 
   /**
+   * Key is the method signature, value is the number of unique instances with
+   * the same signature.
+   */
+  private Map<String, Integer> jsoMethodInstances = new HashMap<String, Integer>();
+
+  /**
    * Contains the set of devirtualizing methods that replace polymorphic calls
    * to Object methods.
    */
@@ -224,10 +230,21 @@
     SourceInfo sourceInfo = jsoType.getSourceInfo().makeChild(SourceOrigin.UNKNOWN);
 
     // Create the new method.
-    String name = polyMethod.getName() + "__devirtual$";
+    String prefix;
+    Integer methodCount;
+    methodCount = jsoMethodInstances.get(polyMethod.getSignature());
+    if (methodCount == null) {
+      prefix = polyMethod.getName();
+      methodCount = 0;
+    } else {
+      prefix = polyMethod.getName() + methodCount;
+      methodCount++;
+    }
+    jsoMethodInstances.put(polyMethod.getSignature(), methodCount);
+    String devirtualName = prefix + "__devirtual$";
     JMethod newMethod =
-        program.createMethod(sourceInfo, name, jsoType, polyMethod.getType(), false, true, true,
-            AccessModifier.PUBLIC, false);
+        program.createMethod(sourceInfo, devirtualName, jsoType, polyMethod.getType(), false, true,
+            true, AccessModifier.PUBLIC, false);
     newMethod.setSynthetic();
 
     // Setup parameters.
@@ -297,4 +314,4 @@
     }
     return staticImpl;
   }
-}
\ No newline at end of file
+}
diff --git a/dev/core/test/com/google/gwt/dev/jjs/impl/JsoDevirtualizerTest.java b/dev/core/test/com/google/gwt/dev/jjs/impl/JsoDevirtualizerTest.java
new file mode 100644
index 0000000..b91092f
--- /dev/null
+++ b/dev/core/test/com/google/gwt/dev/jjs/impl/JsoDevirtualizerTest.java
@@ -0,0 +1,95 @@
+/*
+ * Copyright 2011 Google Inc.
+ * 
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ * 
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+package com.google.gwt.dev.jjs.impl;
+
+import com.google.gwt.core.ext.UnableToCompleteException;
+import com.google.gwt.dev.javac.testing.impl.MockJavaResource;
+import com.google.gwt.dev.jjs.ast.JMethod;
+import com.google.gwt.dev.jjs.ast.JProgram;
+
+/**
+ * Tests for the {@link JsoDevirtualizer} visitor.
+ */
+public class JsoDevirtualizerTest extends OptimizerTestBase {
+
+  /**
+   * JsoDevirtualizer should allow dual Java/JSO implementations of the same
+   * interface, so long as there is only one of each. If there are multiple
+   * methods with the same method name, it should distinguish between them.
+   */
+  public void testDualJsoImpl() throws UnableToCompleteException {
+
+    sourceOracle.addOrReplace(new MockJavaResource("com.google.gwt.lang.Cast") {
+      @Override
+      public CharSequence getContent() {
+        StringBuffer code = new StringBuffer();
+        code.append("package com.google.gwt.lang;");
+        code.append("public class Cast {");
+        code.append("  public static boolean isJavaObject(Object o) { return true; };");
+        code.append("  public static boolean isJavaScriptObject(Object o) { return true; };");
+        code.append("}");
+        return code;
+      }
+    });
+    
+    addSnippetImport("com.google.gwt.lang.Cast");
+    addSnippetImport("com.google.gwt.core.client.JavaScriptObject");
+    
+    addSnippetClassDecl(
+        "interface Iface1 { int a(); }",
+        "static class J1 implements Iface1 {",
+        "  public int a() { return 1; }",
+        "}",
+        "static class Jso1 extends JavaScriptObject implements Iface1 {",
+        "  protected Jso1() { }",
+        "  public final int a() { return 2; }",
+        "  public static native Jso1 create() /*-{ return {} }-*/;",
+        "}",
+        "static interface Iface2 { int a(); }",
+        "static class J2 implements Iface2 {",
+        "  public int a() { return 3; }",
+        "}",
+        "static class Jso2 extends JavaScriptObject implements Iface2 {",
+        "  protected Jso2() { }", "  public final int a() { return 4; }",
+        "  public static native Jso2 create() /*-{ return {} }-*/;", 
+        "}",
+        "static Iface1 val1 = new J1();",
+        "static Iface1 val2 = Jso1.create();",
+        "static Iface2 val3 = new J2();",
+        "static Iface2 val4 = Jso2.create();");        
+
+    StringBuilder code = new StringBuilder();    
+    code.append("int result = val1.a() + val2.a() + val3.a() + val4.a();");
+            
+    // The salient point in the results below is that the JSO method used for
+    // val1 and val1 has a different name the method used for val2 and val3.
+    StringBuffer expected = new StringBuffer();
+    expected.append("int result = ");
+    expected.append("JavaScriptObject.a__devirtual$(EntryPoint.val1) + ");
+    expected.append("JavaScriptObject.a__devirtual$(EntryPoint.val2) + ");
+    expected.append("JavaScriptObject.a0__devirtual$(EntryPoint.val3) + ");
+    expected.append("JavaScriptObject.a0__devirtual$(EntryPoint.val4);");
+    
+    optimize("void", code.toString()).intoString(expected.toString());
+  }
+
+  @Override
+  protected boolean optimizeMethod(JProgram program, JMethod method) {
+    JsoDevirtualizer.exec(program);
+    return true;
+  }
+}