replace put with putIfAbsent to make fundbugs more happy
diff --git a/src/main/org/codehaus/groovy/runtime/ConversionHandler.java b/src/main/org/codehaus/groovy/runtime/ConversionHandler.java
index 6791cb0..cbf69e7 100644
--- a/src/main/org/codehaus/groovy/runtime/ConversionHandler.java
+++ b/src/main/org/codehaus/groovy/runtime/ConversionHandler.java
@@ -93,7 +93,7 @@
             Object handle = handleCache.get(method);

             if (handle == null) {

                 handle = plugin.getInvokeSpecialHandle(method, proxy);

-                handleCache.put(method, handle);

+                handleCache.putIfAbsent(method, handle);

             }

             return plugin.invokeHandle(handle, args);

         }