[stubs] Fix flaky failures of cctest/test-code-stub-assembler/SeededNumberDictionaryLookup.

test/cctest/test-code-stub-assembler.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/base/utils/random-number-generator.h"
 #include "src/ic/stub-cache.h"
 #include "src/interface-descriptors.h"
 #include "src/isolate.h"
 #include "test/cctest/compiler/function-tester.h"
 
@@ skipping 678 matching lines @@
   uint32_t keys[kKeysCount];
 
   Handle<Object> fake_value(Smi::FromInt(42), isolate);
   PropertyDetails fake_details = PropertyDetails::Empty();
 
   base::RandomNumberGenerator rand_gen(FLAG_random_seed);
 
   for (int i = 0; i < kKeysCount; i++) {
     int random_key = rand_gen.NextInt(Smi::kMaxValue);
     keys[i] = static_cast<uint32_t>(random_key);
+    if (dictionary->FindEntry(keys[i]) != Dictionary::kNotFound) continue;
 
     dictionary = Dictionary::Add(dictionary, keys[i], fake_value, fake_details);
   }
 
   // Now try querying existing keys.
   for (int i = 0; i < kKeysCount; i++) {
     int entry = dictionary->FindEntry(keys[i]);
     CHECK_NE(Dictionary::kNotFound, entry);
 
     Handle<Object> key(Smi::FromInt(keys[i]), isolate);
@@ skipping 687 matching lines @@
 
     Handle<Code> expected_handler(handler, isolate);
     ft.CheckTrue(receiver, name, expected_handler);
   }
   // Ensure we performed both kind of queries.
   CHECK(queried_existing && queried_non_existing);
 }
 
 }  // namespace internal
 }  // namespace v8