diff --git a/testable-agent/src/main/java/com/alibaba/testable/agent/handler/TestClassHandler.java b/testable-agent/src/main/java/com/alibaba/testable/agent/handler/TestClassHandler.java
index 907441c..fce19cd 100644
--- a/testable-agent/src/main/java/com/alibaba/testable/agent/handler/TestClassHandler.java
+++ b/testable-agent/src/main/java/com/alibaba/testable/agent/handler/TestClassHandler.java
@@ -4,9 +4,11 @@ import com.alibaba.testable.agent.constant.ConstPool;
 import com.alibaba.testable.agent.tool.ImmutablePair;
 import com.alibaba.testable.agent.util.AnnotationUtil;
 import com.alibaba.testable.agent.util.ClassUtil;
+import com.alibaba.testable.core.util.LogUtil;
 import org.objectweb.asm.tree.*;
 
 import java.util.ArrayList;
+import java.util.Iterator;
 import java.util.List;
 
 import static com.alibaba.testable.agent.util.ClassUtil.toDotSeparateFullClassName;
@@ -36,11 +38,19 @@ public class TestClassHandler extends BaseClassHandler {
      */
     @Override
     protected void transform(ClassNode cn) {
-        for (MethodNode m : cn.methods) {
-            transformMethod(cn, m);
+        Iterator<FieldNode> iterator = cn.fields.iterator();
+        if (iterator.hasNext()) {
+            if (ConstPool.TESTABLE_INJECT_REF.equals(iterator.next().name)) {
+                // avoid duplicate injection
+                LogUtil.verbose("Duplicate injection found, ignore " + cn.name);
+                return;
+            }
         }
         cn.fields.add(new FieldNode(ACC_PUBLIC | ACC_STATIC, ConstPool.TESTABLE_INJECT_REF,
             ClassUtil.toByteCodeClassName(cn.name), null, null));
+        for (MethodNode m : cn.methods) {
+            transformMethod(cn, m);
+        }
     }
 
     private void transformMethod(ClassNode cn, MethodNode mn) {
diff --git a/testable-agent/src/main/java/com/alibaba/testable/agent/util/ClassUtil.java b/testable-agent/src/main/java/com/alibaba/testable/agent/util/ClassUtil.java
index 8a8aaa8..0d0cec1 100644
--- a/testable-agent/src/main/java/com/alibaba/testable/agent/util/ClassUtil.java
+++ b/testable-agent/src/main/java/com/alibaba/testable/agent/util/ClassUtil.java
@@ -1,14 +1,12 @@
 package com.alibaba.testable.agent.util;
 
 import com.alibaba.testable.agent.constant.ConstPool;
-import com.alibaba.testable.agent.tool.ComparableWeakRef;
-import org.objectweb.asm.ClassReader;
-import org.objectweb.asm.tree.AnnotationNode;
-import org.objectweb.asm.tree.ClassNode;
 import org.objectweb.asm.tree.MethodInsnNode;
-import org.objectweb.asm.tree.MethodNode;
 
-import java.util.*;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
 
 import static org.objectweb.asm.Opcodes.INVOKESTATIC;
 
diff --git a/testable-core/src/main/java/com/alibaba/testable/core/util/LogUtil.java b/testable-core/src/main/java/com/alibaba/testable/core/util/LogUtil.java
index 97431cc..2e82b94 100644
--- a/testable-core/src/main/java/com/alibaba/testable/core/util/LogUtil.java
+++ b/testable-core/src/main/java/com/alibaba/testable/core/util/LogUtil.java
@@ -30,7 +30,7 @@ public class LogUtil {
     }
 
     private static LogLevel defaultLogLevel = LogLevel.LEVEL_WARN;
-    private static LogLevel currentLogLevel;
+    private static LogLevel currentLogLevel = LogLevel.LEVEL_WARN;
 
     public static void verbose(String msg, Object... args) {
         if (currentLogLevel.level >= LogLevel.LEVEL_VERBOSE.level) {