Index: src/main/org/codehaus/groovy/classgen/AsmClassGenerator.java
===================================================================
--- src/main/org/codehaus/groovy/classgen/AsmClassGenerator.java	(revision 20330)
+++ src/main/org/codehaus/groovy/classgen/AsmClassGenerator.java	(working copy)
@@ -4084,9 +4084,14 @@
     }
 
     private void execMethodAndStoreForSubscriptOperator(String method, Expression expression) {
+    	execMethodAndStoreForSubscriptOperator(method, expression, null);
+    }
+    
+    private void execMethodAndStoreForSubscriptOperator(String method, Expression expression,
+    		Expression getAtResultExp) {
         // execute method
         makeCallSite(
-                expression,
+        		(getAtResultExp == null ? expression : getAtResultExp),
                 method,
                 MethodCallExpression.NO_ARGUMENTS,
                 false, false, false, false);
@@ -4132,20 +4137,54 @@
     }
 
     protected void evaluatePostfixMethod(String method, Expression expression) {
+    	// GROOVY-4246: arr[rand()]++ should evaulate rand() only once and reuse its result
+    	boolean getAtOp = false;
+    	BinaryExpression be = null;
+    	Expression getAtResultExp = null;
+    	String varName = "tmp_postfix_" + method;
+    	final int idx = compileStack.defineTemporaryVariable(varName, false);
+    	
+    	if(expression instanceof BinaryExpression) {
+    		be = (BinaryExpression) expression;
+    		if (be.getOperation().getType() == Types.LEFT_SQUARE_BRACKET) {
+    			getAtOp = true;
+    			be.getRightExpression().visit(this); // execute subscript exp
+    			mv.visitVarInsn(ASTORE, idx); // store it in a local variable
+    			BytecodeExpression newRightExp = new BytecodeExpression() {
+    				public void visit(MethodVisitor mv) {
+    					mv.visitVarInsn(ALOAD, idx);
+    				}
+    			};
+    			// change the subscript exp to pick up the local var so that the orig exp is not re-executed
+    			be.setRightExpression(newRightExp);
+    		}
+    		
+    	}
         // load
         expression.visit(this);
 
         // save value for later
-        int tempIdx = compileStack.defineTemporaryVariable("postfix_" + method, true);
+        final int tempIdx = compileStack.defineTemporaryVariable("postfix_" + method, true);
 
+        if(getAtOp) {
+        	 // exp to allow reuse of binary exp already evaluated so that following putAt does not
+        	// executed the getAt() again
+        	getAtResultExp = new BytecodeExpression() {
+        		public void visit(MethodVisitor mv) {
+        			mv.visitVarInsn(ALOAD, tempIdx);
+        		}
+        	};
+        }
+        
         // execute Method
-        execMethodAndStoreForSubscriptOperator(method, expression);
+        execMethodAndStoreForSubscriptOperator(method, expression, getAtResultExp);
         // remove the result of the method call
         mv.visitInsn(POP);
 
         //reload saved value
         mv.visitVarInsn(ALOAD, tempIdx);
         compileStack.removeVar(tempIdx);
+        compileStack.removeVar(idx);
     }
 
     protected void evaluateInstanceof(BinaryExpression expression) {
Index: src/test/groovy/bugs/Groovy4246Bug.groovy
===================================================================
--- src/test/groovy/bugs/Groovy4246Bug.groovy	(revision 0)
+++ src/test/groovy/bugs/Groovy4246Bug.groovy	(revision 0)
@@ -0,0 +1,44 @@
+/*
+ * Copyright 2003-2010 the original author or authors.
+ *
+ * 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 groovy.bugs
+
+class Groovy4246Bug extends GroovyTestCase {
+    void testPostFixExpEvaluations() {
+        assertScript """
+            class Bug4246 {
+                int randCallCount = 0
+                static void main(args) {
+                    new Bug4246()    
+                }
+            
+                Bug4246() {
+                    def num = 10
+                    def arr = [0, 0, 0]
+                    for (def i = 0; i < num; i++) {
+                        arr[rand()]++
+                    }
+                    assert (arr[0] + arr[1] + arr[2] == num)
+                    assert randCallCount == 10
+                }
+            
+                int rand() {
+                    randCallCount++
+                    return new Random().nextInt(3)    
+                }
+            }
+        """
+    }
+}