diff --git a/src/main/java/com/android/tools/r8/naming/Minifier.java b/src/main/java/com/android/tools/r8/naming/Minifier.java
index 5333125..8f923b0 100644
--- a/src/main/java/com/android/tools/r8/naming/Minifier.java
+++ b/src/main/java/com/android/tools/r8/naming/Minifier.java
@@ -24,6 +24,7 @@
 import com.android.tools.r8.utils.InternalOptions;
 import com.android.tools.r8.utils.Timing;
 import com.google.common.collect.ImmutableMap;
+import java.util.HashMap;
 import java.util.IdentityHashMap;
 import java.util.Map;
 import java.util.Set;
@@ -91,6 +92,8 @@
     private final AppInfo appInfo;
     private final Map<String, String> packageRenaming;
     private final Map<DexItem, DexString> renaming = new IdentityHashMap<>();
+    // This set is only used for asserting no duplicated names.
+    private final Map<DexString, DexType> renamedTypesForVerification;
 
     private MinifiedRenaming(
         ClassRenaming classRenaming,
@@ -103,6 +106,10 @@
       renaming.putAll(methodRenaming.renaming);
       renaming.putAll(methodRenaming.callSiteRenaming);
       renaming.putAll(fieldRenaming.renaming);
+      renamedTypesForVerification = new HashMap<>();
+      for (Map.Entry<DexType, DexString> entry : classRenaming.classRenaming.entrySet()) {
+        renamedTypesForVerification.put(entry.getValue(), entry.getKey());
+      }
     }
 
     @Override
@@ -112,7 +119,18 @@
 
     @Override
     public DexString lookupDescriptor(DexType type) {
-      return renaming.getOrDefault(type, type.descriptor);
+      DexString dexString = renaming.get(type);
+      if (dexString != null) {
+        return dexString;
+      }
+      assert type.isPrimitiveType()
+              || type.isVoidType()
+              || !renamedTypesForVerification.containsKey(type.descriptor)
+          : "Duplicate minified type '"
+              + type.descriptor
+              + "' already mapped for: "
+              + renamedTypesForVerification.get(type.descriptor);
+      return type.descriptor;
     }
 
     @Override
diff --git a/src/test/java/com/android/tools/r8/naming/MinifierTest.java b/src/test/java/com/android/tools/r8/naming/MinifierTest.java
index 473ca53..adf74ce 100644
--- a/src/test/java/com/android/tools/r8/naming/MinifierTest.java
+++ b/src/test/java/com/android/tools/r8/naming/MinifierTest.java
@@ -3,13 +3,16 @@
 // BSD-style license that can be found in the LICENSE file.
 package com.android.tools.r8.naming;
 
+import static junit.framework.TestCase.assertTrue;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotEquals;
+import static org.junit.Assert.fail;
 
 import com.android.tools.r8.graph.DexField;
 import com.android.tools.r8.graph.DexItemFactory;
 import com.android.tools.r8.graph.DexMethod;
 import com.android.tools.r8.graph.DexType;
+import com.android.tools.r8.naming.Minifier.MinifiedRenaming;
 import com.android.tools.r8.utils.ListUtils;
 import com.android.tools.r8.utils.Timing;
 import java.nio.file.Paths;
@@ -40,6 +43,25 @@
     inspection.accept(dexItemFactory, naming);
   }
 
+  @Test
+  public void ensureClassesAddedToRenamingOrNoClashTest() throws Exception {
+    MinifiedRenaming naming =
+        (MinifiedRenaming) runMinifier(ListUtils.map(keepRulesFiles, Paths::get));
+    // Create a type that exists.
+    String existingType = "La/c;";
+    DexType d = dexItemFactory.createType(existingType);
+    try {
+      naming.lookupDescriptor(d);
+    } catch (AssertionError ae) {
+      assertTrue(
+          ae.getMessage()
+              .startsWith(
+                  "Duplicate minified type '" + existingType + "' already mapped for: naming001."));
+      return;
+    }
+    fail("Should have thrown an error.");
+  }
+
   @Parameters(name = "test: {0} keep: {1}")
   public static Collection<Object[]> data() {
     List<String> tests = Arrays.asList("naming001");