diff --git a/client/src/main/scala/org/apache/celeborn/client/commit/LegacySkewHandlingPartitionValidator.scala b/client/src/main/scala/org/apache/celeborn/client/commit/LegacySkewHandlingPartitionValidator.scala
index 12028a47776..cbb1be5951c 100644
--- a/client/src/main/scala/org/apache/celeborn/client/commit/LegacySkewHandlingPartitionValidator.scala
+++ b/client/src/main/scala/org/apache/celeborn/client/commit/LegacySkewHandlingPartitionValidator.scala
@@ -125,8 +125,13 @@ class LegacySkewHandlingPartitionValidator extends AbstractPartitionCompleteness
           return (false, errorMessage)
         }
 
-        // Process new range
-        subRangeToCommitMetadataMap.put(rangeKey, actualCommitMetadata)
+        // Process new range.
+        // Store a defensive copy so that subsequent merges into currentCommitMetadataForReducer
+        // (which mutate the CommitMetadata object in-place via addCommitData) do not corrupt
+        // the per-range value kept in subRangeToCommitMetadataMap.
+        subRangeToCommitMetadataMap.put(
+          rangeKey,
+          new CommitMetadata(actualCommitMetadata.getChecksum, actualCommitMetadata.getBytes))
         currentCommitMetadataForReducer.merge(
           partitionId,
           actualCommitMetadata,
diff --git a/client/src/test/scala/org/apache/celeborn/client/commit/LegacySkewHandlingPartitionValidatorTest.scala b/client/src/test/scala/org/apache/celeborn/client/commit/LegacySkewHandlingPartitionValidatorTest.scala
new file mode 100644
index 00000000000..25cf4190379
--- /dev/null
+++ b/client/src/test/scala/org/apache/celeborn/client/commit/LegacySkewHandlingPartitionValidatorTest.scala
@@ -0,0 +1,98 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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 org.apache.celeborn.client.commit
+
+import org.scalatest.matchers.should.Matchers.{convertToAnyShouldWrapper, include, not}
+
+import org.apache.celeborn.CelebornFunSuite
+import org.apache.celeborn.common.CommitMetadata
+
+class LegacySkewHandlingPartitionValidatorTest extends CelebornFunSuite {
+
+  // Reproduces the object-aliasing bug:
+  // When sub-range A [0,585) is processed first, its CommitMetadata object (OBJ_A) is stored
+  // in both subRangeToCommitMetadataMap and currentCommitMetadataForReducer as the same
+  // reference. When sibling sub-range B [585,910) arrives, metadataMergeBiFunction mutates
+  // OBJ_A in-place via addCommitData(). The TreeMap entry for [0,585) now silently holds
+  // bytes(A) + bytes(B) instead of bytes(A). Any retry of sub-range A sends bytes(A) but the
+  // validator finds bytes(A)+bytes(B) stored, causing a false mismatch.
+  test("sibling sub-range arrival must not corrupt the stored metadata of an earlier sub-range") {
+    val validator = new LegacySkewHandlingPartitionValidator
+    val partitionId = 387
+    val numMappers = 910
+
+    val bytesA = 650884788L
+    val bytesB = 1158249332L
+    val metadataA = new CommitMetadata(1, bytesA)
+    val metadataB = new CommitMetadata(2, bytesB)
+
+    // Sub-range [0, 585) arrives first and is stored successfully.
+    val (ok1, _) = validator.processSubPartition(partitionId, 0, 585, metadataA, numMappers)
+    ok1 shouldBe true
+
+    // Sibling sub-range [585, 910) arrives. Before the fix, this mutates the object stored
+    // for [0, 585), corrupting it from bytesA to bytesA+bytesB.
+    val (ok2, _) = validator.processSubPartition(partitionId, 585, 910, metadataB, numMappers)
+    ok2 shouldBe true
+
+    // Now simulate a task retry: sub-range [0, 585) is submitted again with the same correct
+    // metadata. Before the fix this fails because the stored value is now bytesA+bytesB.
+    val metadataARetry = new CommitMetadata(1, bytesA)
+    val (ok3, msg3) =
+      validator.processSubPartition(partitionId, 0, 585, metadataARetry, numMappers)
+    ok3 shouldBe true
+    msg3 should not include "not matching"
+  }
+
+  test("two sub-ranges complete a partition when bytes sum matches expected") {
+    val validator = new LegacySkewHandlingPartitionValidator
+    val partitionId = 1
+    val numMappers = 10
+
+    val (ok1, _) =
+      validator.processSubPartition(partitionId, 0, 5, new CommitMetadata(0, 100), numMappers)
+    ok1 shouldBe true
+    validator.isPartitionComplete(partitionId) shouldBe false
+
+    val (ok2, _) =
+      validator.processSubPartition(partitionId, 5, 10, new CommitMetadata(0, 200), numMappers)
+    ok2 shouldBe true
+    validator.isPartitionComplete(partitionId) shouldBe true
+
+    validator.currentCommitMetadata(partitionId).getBytes shouldBe 300L
+  }
+
+  test("retry of a sub-range after all siblings completed must succeed") {
+    val validator = new LegacySkewHandlingPartitionValidator
+    val partitionId = 5
+    val numMappers = 4
+
+    // Process 4 non-overlapping sub-ranges of size 1 each
+    validator.processSubPartition(partitionId, 0, 1, new CommitMetadata(1, 100), numMappers)
+    validator.processSubPartition(partitionId, 1, 2, new CommitMetadata(2, 200), numMappers)
+    validator.processSubPartition(partitionId, 2, 3, new CommitMetadata(3, 300), numMappers)
+    validator.processSubPartition(partitionId, 3, 4, new CommitMetadata(4, 400), numMappers)
+    validator.isPartitionComplete(partitionId) shouldBe true
+
+    // Retry of the first sub-range — must not be poisoned by sibling accumulation
+    val (ok, msg) =
+      validator.processSubPartition(partitionId, 0, 1, new CommitMetadata(1, 100), numMappers)
+    ok shouldBe true
+    msg should not include "not matching"
+  }
+}