Added the validation to avoid override existent function.

json-path/src/main/java/com/jayway/jsonpath/internal/function/PathFunctionFactory.java

@@ -1,7 +1,6 @@
 package com.jayway.jsonpath.internal.function;
 
 import com.jayway.jsonpath.InvalidPathException;
-import com.jayway.jsonpath.internal.Path;
 import com.jayway.jsonpath.internal.function.json.Append;
 import com.jayway.jsonpath.internal.function.json.KeySetFunction;
 import com.jayway.jsonpath.internal.function.numeric.Average;
@@ -16,7 +15,6 @@ import com.jayway.jsonpath.internal.function.text.Concatenate;
 import com.jayway.jsonpath.internal.function.text.Length;
 
 import java.io.InvalidClassException;
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 
@@ -89,6 +87,9 @@ public class PathFunctionFactory {
     }
 
     public static void addCustomFunction(String name,Class function) throws InvalidClassException {
+        if(FUNCTIONS.containsKey(name)){
+            throw new InvalidPathException("Function with name: " + name + " already exists");
+        }
         if (!PathFunction.class.isAssignableFrom(function)){
             throw new InvalidClassException("Function with name: " + name + "must be a instance of PathFunction class");
         }

json-path/src/test/java/com/jayway/jsonpath/internal/function/CustomFunctionTest.java

@@ -2,17 +2,15 @@ package com.jayway.jsonpath.internal.function;
 
 import com.jayway.jsonpath.Configuration;
 import com.jayway.jsonpath.Configurations;
-import com.jayway.jsonpath.internal.EvaluationContext;
-import com.jayway.jsonpath.internal.PathRef;
+import com.jayway.jsonpath.InvalidPathException;
 import org.junit.jupiter.api.Test;
 
 import java.io.InvalidClassException;
-import java.util.List;
 
 import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
-public class CustomFunctionTest extends BaseFunctionTest {
+class CustomFunctionTest extends BaseFunctionTest {
 
     private Configuration conf = Configurations.JSON_SMART_CONFIGURATION;
 
@@ -30,4 +28,10 @@ public class CustomFunctionTest extends BaseFunctionTest {
        assertDoesNotThrow(()->PathFunctionFactory.addCustomFunction("toUpperCase",ToUpperCase.class));
        verifyTextFunction(conf,"$['text'][0].toUpperCase()","A");
     }
+
+    @Test
+    void testAddExistentFunction(){
+        assertThrows(InvalidPathException.class, () -> PathFunctionFactory.addCustomFunction("avg",
+                ToUpperCase.class));
+    }
 }