diff --git a/config/pmd.xml b/config/pmd.xml
index 09964100a..61fc1f9ce 100644
--- a/config/pmd.xml
+++ b/config/pmd.xml
@@ -113,7 +113,6 @@
     <exclude name="ConfusingTernary"/>
     <!-- extra final modifier does not make code more secure in that cases-->
     <exclude name="ImmutableField"/>
-    <exclude name="AvoidSynchronizedAtMethodLevel"/>
     <exclude name="MissingBreakInSwitch"/>
     <exclude name="UseNotifyAllInsteadOfNotify"/>
     <exclude name="AvoidInstanceofChecksInCatchClause"/>
diff --git a/src/main/java/com/puppycrawl/tools/checkstyle/checks/UniquePropertiesCheck.java b/src/main/java/com/puppycrawl/tools/checkstyle/checks/UniquePropertiesCheck.java
index ce40a1f9f..417d8b02a 100644
--- a/src/main/java/com/puppycrawl/tools/checkstyle/checks/UniquePropertiesCheck.java
+++ b/src/main/java/com/puppycrawl/tools/checkstyle/checks/UniquePropertiesCheck.java
@@ -138,14 +138,16 @@ public class UniquePropertiesCheck extends AbstractFileSetCheck
                 .create();
 
         @Override
-        public synchronized Object put(Object key, Object value)
+        public Object put(Object key, Object value)
         {
-            final Object oldValue = super.put(key, value);
-            if (oldValue != null && key instanceof String) {
-                final String keyString = (String) key;
-                duplicatedStrings.add(keyString);
+            synchronized (this) {
+                final Object oldValue = super.put(key, value);
+                if (oldValue != null && key instanceof String) {
+                    final String keyString = (String) key;
+                    duplicatedStrings.add(keyString);
+                }
+                return oldValue;
             }
-            return oldValue;
         }
 
         public Multiset<String> getDuplicatedStrings()