diff --git a/pitest/src/main/java/org/pitest/mutationtest/engine/gregor/GregorMutater.java b/pitest/src/main/java/org/pitest/mutationtest/engine/gregor/GregorMutater.java
index cdd821f07..8343ecbc6 100644
--- a/pitest/src/main/java/org/pitest/mutationtest/engine/gregor/GregorMutater.java
+++ b/pitest/src/main/java/org/pitest/mutationtest/engine/gregor/GregorMutater.java
@@ -19,14 +19,14 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.HashMap;
-import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
-import java.util.Set;
 import java.util.function.Function;
 import java.util.function.Predicate;
+import java.util.stream.Collectors;
 
 import org.objectweb.asm.ClassReader;
 import org.objectweb.asm.ClassWriter;
@@ -46,13 +46,13 @@ public class GregorMutater implements Mutater {
   private final Map<String, String>       computeCache   = new HashMap<>();
   private final Predicate<MethodInfo>     filter;
   private final ClassByteArraySource      byteSource;
-  private final Set<MethodMutatorFactory> mutators       = new HashSet<>();
+  private final List<MethodMutatorFactory> mutators;
 
   public GregorMutater(final ClassByteArraySource byteSource,
       final Predicate<MethodInfo> filter,
       final Collection<MethodMutatorFactory> mutators) {
     this.filter = filter;
-    this.mutators.addAll(mutators);
+    this.mutators = orderAndDeDuplicate(mutators);
     this.byteSource = byteSource;
   }
 
@@ -134,4 +134,13 @@ private static Predicate<MethodInfo> isGeneratedEnumMethod() {
     return MethodInfo::isGeneratedEnumMethod;
   }
 
+  private List<MethodMutatorFactory> orderAndDeDuplicate(Collection<MethodMutatorFactory> mutators) {
+    // deduplication is based on object identity, so dubious that this adds any value
+    // however left in place for now to replicate HashSet behaviour
+    return mutators.stream()
+            .distinct()
+            .sorted(Comparator.comparing(MethodMutatorFactory::getGloballyUniqueId))
+            .collect(Collectors.toList());
+  }
+
 }
diff --git a/pitest/src/main/java/org/pitest/mutationtest/engine/gregor/MutatingClassVisitor.java b/pitest/src/main/java/org/pitest/mutationtest/engine/gregor/MutatingClassVisitor.java
index a84a6ea65..7b5cc8233 100644
--- a/pitest/src/main/java/org/pitest/mutationtest/engine/gregor/MutatingClassVisitor.java
+++ b/pitest/src/main/java/org/pitest/mutationtest/engine/gregor/MutatingClassVisitor.java
@@ -14,9 +14,7 @@
  */
 package org.pitest.mutationtest.engine.gregor;
 
-import java.util.Collection;
-import java.util.HashSet;
-import java.util.Set;
+import java.util.List;
 import java.util.function.Predicate;
 
 import org.objectweb.asm.ClassVisitor;
@@ -32,15 +30,15 @@ class MutatingClassVisitor extends ClassVisitor {
 
   private final Predicate<MethodInfo>    filter;
   private final ClassContext              context;
-  private final Set<MethodMutatorFactory> methodMutators = new HashSet<>();
+  private final List<MethodMutatorFactory> methodMutators;
 
   MutatingClassVisitor(final ClassVisitor delegateClassVisitor,
       final ClassContext context, final Predicate<MethodInfo> filter,
-      final Collection<MethodMutatorFactory> mutators) {
+      final List<MethodMutatorFactory> mutators) {
     super(ASMVersion.ASM_VERSION, delegateClassVisitor);
     this.context = context;
     this.filter = filter;
-    this.methodMutators.addAll(mutators);
+    this.methodMutators = mutators;
   }
 
   @Override