diff --git a/sfm-jmh/src/main/java/org/simpleflatmapper/csv/DiscriminatorBenchmark.java b/sfm-jmh/src/main/java/org/simpleflatmapper/csv/DiscriminatorBenchmark.java
index 834451f35..0185c1e39 100644
--- a/sfm-jmh/src/main/java/org/simpleflatmapper/csv/DiscriminatorBenchmark.java
+++ b/sfm-jmh/src/main/java/org/simpleflatmapper/csv/DiscriminatorBenchmark.java
@@ -6,6 +6,7 @@
 import org.openjdk.jmh.annotations.State;
 import org.openjdk.jmh.annotations.TearDown;
 import org.openjdk.jmh.infra.Blackhole;
+import org.simpleflatmapper.util.Consumer;
 
 import java.io.File;
 import java.io.FileOutputStream;
@@ -96,6 +97,76 @@ public void testWithDiscriminator(Blackhole blackhole) throws IOException {
         });
     }
 
+    @Benchmark
+    public void testWithDiscriminatorWithKeys(Blackhole blackhole) throws IOException {
+        CsvParser.mapWith(
+                CsvMapperFactory.newInstance()
+                        .discriminator(A.class, r -> r.getString(2), b -> {
+                            for(int i = 1; i <= 40; i++) {
+                                try {
+                                    b.when("typea" + i, Class.forName("org.simpleflatmapper.csv.DiscriminatorBenchmark$A" + i));
+                                } catch (ClassNotFoundException e) {
+                                    throw new Error(e);
+                                }
+                            }
+                        })
+                        .discriminator(B.class, r -> r.getString(4), b -> {
+                            for(int i = 1; i <= 20; i++) {
+                                try {
+                                    b.when("typeb" + i, Class.forName("org.simpleflatmapper.csv.DiscriminatorBenchmark$B" + i));
+                                } catch (ClassNotFoundException e) {
+                                    throw new Error(e);
+                                }
+                            }
+                        })
+                        .addKeys("id", "b_id", "b_c_id")
+                        .newMapper(A.class)
+        ).stream(file, s -> {
+            s.forEach(blackhole::consume);
+            return null;
+        });
+    }
+
+    public void withDiscriminatorWithKeys(Consumer<A> aConsumer) throws IOException {
+        CsvParser.mapWith(
+                CsvMapperFactory.newInstance()
+                        .discriminator(A.class, r -> r.getString(2), b -> {
+                            for(int i = 1; i <= 40; i++) {
+                                try {
+                                    b.when("typea" + i, Class.forName("org.simpleflatmapper.csv.DiscriminatorBenchmark$A" + i));
+                                } catch (ClassNotFoundException e) {
+                                    throw new Error(e);
+                                }
+                            }
+                        })
+                        .discriminator(B.class, r -> r.getString(4), b -> {
+                            for(int i = 1; i <= 20; i++) {
+                                try {
+                                    b.when("typeb" + i, Class.forName("org.simpleflatmapper.csv.DiscriminatorBenchmark$B" + i));
+                                } catch (ClassNotFoundException e) {
+                                    throw new Error(e);
+                                }
+                            }
+                        })
+                        .addKeys("id", "b_id", "b_c_id")
+                        .newMapper(A.class)
+        ).stream(file, s -> {
+            s.forEach(aConsumer);
+            return null;
+        });
+    }
+
+
+    public static void main(String[] args) throws IOException {
+        DiscriminatorBenchmark db = new DiscriminatorBenchmark();
+        db.setUp();
+
+        db.withDiscriminatorWithKeys(v -> {
+            System.out.println("v = " + v);
+        });
+
+        db.tearDown();
+    }
 
     public static class A {
         public final Integer id;
@@ -313,9 +384,6 @@ public B20(Integer id, String type, C c) {
         }
     }
 
-    public static void main(String[] args) {
-        System.out.println("A1.class.getName() = " + A1.class.getName());
-    }
     
 
     public static class A1 extends A
@@ -598,4 +666,6 @@ public A40(Integer id, String name, String type, B b) {
             super(id, name, type, b);
         }
     }
+
 }
+
diff --git a/sfm-map/src/main/java/org/simpleflatmapper/map/context/KeyAndPredicate.java b/sfm-map/src/main/java/org/simpleflatmapper/map/context/KeyAndPredicate.java
index 3261250e2..c44025086 100644
--- a/sfm-map/src/main/java/org/simpleflatmapper/map/context/KeyAndPredicate.java
+++ b/sfm-map/src/main/java/org/simpleflatmapper/map/context/KeyAndPredicate.java
@@ -16,11 +16,29 @@ public boolean test(S s) {
         return predicate == null ? true : predicate.test(s);
     }
 
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+
+        KeyAndPredicate<?, ?> that = (KeyAndPredicate<?, ?>) o;
+
+        if (!key.equals(that.key)) return false;
+        return predicate != null ? predicate.equals(that.predicate) : that.predicate == null;
+    }
+
+    @Override
+    public int hashCode() {
+        int result = key.hashCode();
+        result = 31 * result + (predicate != null ? predicate.hashCode() : 0);
+        return result;
+    }
+
     public KeyAndPredicate<S, K> mergeWith(KeyAndPredicate<S, K> keyAndPredicate) {
         if (!this.key.equals(keyAndPredicate.key)) throw new IllegalArgumentException();
         
         if (this.predicate == null || keyAndPredicate.predicate == null) { // null is equivalent to true
-            return new KeyAndPredicate<S, K>(key, null);
+            return this;
         }
         
         if (this.predicate.equals(keyAndPredicate.predicate)) {
diff --git a/sfm-map/src/main/java/org/simpleflatmapper/map/context/MappingContextFactoryBuilder.java b/sfm-map/src/main/java/org/simpleflatmapper/map/context/MappingContextFactoryBuilder.java
index 1ff4b1037..3a3549748 100644
--- a/sfm-map/src/main/java/org/simpleflatmapper/map/context/MappingContextFactoryBuilder.java
+++ b/sfm-map/src/main/java/org/simpleflatmapper/map/context/MappingContextFactoryBuilder.java
@@ -75,6 +75,13 @@ public Predicate<S> nullChecker() {
     }
 
     public MappingContextFactoryBuilder<S, K> newBuilder(List<KeyAndPredicate<S, K>> subKeys, PropertyMeta<?, ?> owner) {
+        // look for duplicate 
+        for(MappingContextFactoryBuilder<S, K> child : children) {
+            if (!subKeys.isEmpty() && child.keys.equals(subKeys) && owner.equals(owner)) {
+                return child;
+            }
+        }
+        
         MappingContextFactoryBuilder<S, K> subBuilder = new MappingContextFactoryBuilder<S, K>(counter, subKeys, keySourceGetter, this, owner);
         children.add(subBuilder);
         return subBuilder;
diff --git a/sfm-map/src/main/java/org/simpleflatmapper/map/mapper/DefaultConstantSourceMapperBuilder.java b/sfm-map/src/main/java/org/simpleflatmapper/map/mapper/DefaultConstantSourceMapperBuilder.java
index 53536afb1..30f52e671 100644
--- a/sfm-map/src/main/java/org/simpleflatmapper/map/mapper/DefaultConstantSourceMapperBuilder.java
+++ b/sfm-map/src/main/java/org/simpleflatmapper/map/mapper/DefaultConstantSourceMapperBuilder.java
@@ -1017,18 +1017,9 @@ public void accept(Type type, PropertyMeta<?, ?> propertyMeta) {
                     }
                 }).not;
             
+            if (not.isEmpty()) return null;
             
-            return new Predicate<S>() {
-                @Override
-                public boolean test(S s) {
-                    for(Predicate<? super S> p : not) {
-                        if (p.test(s)) {
-                            return false;
-                        }
-                    }
-                    return true;
-                }
-            };
+            return new DiscriminatorKeyPredicate<>(not);
                     
         } else if (propertyMeta.isSubProperty()) {
             SubPropertyMeta subPropertyMeta = (SubPropertyMeta) propertyMeta;
@@ -1391,4 +1382,22 @@ public static String getterNotFoundErrorMessage(PropertyMapping propertyMapping)
                 + " path " + currentPath
                 + ". See " + CSFM_GETTER_NOT_FOUND.toUrl();
     }
+
+    private static class DiscriminatorKeyPredicate<S> implements Predicate<S> {
+        private final List<Predicate<? super S>> not;
+
+        public DiscriminatorKeyPredicate(List<Predicate<? super S>> not) {
+            this.not = not;
+        }
+
+        @Override
+        public boolean test(S s) {
+            for(Predicate<? super S> p : not) {
+                if (p.test(s)) {
+                    return false;
+                }
+            }
+            return true;
+        }
+    }
 }
\ No newline at end of file