diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
index ddfe80443d561..b99cbd0750ce4 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
@@ -214,13 +214,17 @@ abstract class Optimizer(catalogManager: CatalogManager)
       OptimizeSubqueries,
       OptimizeOneRowRelationSubquery),
     Batch("Replace Operators", fixedPoint,
+      // SPARK-51262: ReplaceDeduplicateWithAggregate must run before RewriteExceptAll because
+      // it replaces Deduplicate with Aggregate(First(...)), creating new attribute exprIds.
+      // If RewriteExceptAll runs first, its Generate node captures stale exprIds that no
+      // longer exist after the Deduplicate-to-Aggregate rewrite.
+      ReplaceDeduplicateWithAggregate,
       RewriteExceptAll,
       RewriteIntersectAll,
       ReplaceIntersectWithSemiJoin,
       ReplaceExceptWithFilter,
       ReplaceExceptWithAntiJoin,
-      ReplaceDistinctWithAggregate,
-      ReplaceDeduplicateWithAggregate),
+      ReplaceDistinctWithAggregate),
     Batch("Aggregate", fixedPoint,
       RemoveLiteralFromGroupExpressions,
       RemoveRepetitionFromGroupExpressions),
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/DataFrameSetOperationsSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/DataFrameSetOperationsSuite.scala
index e65942689bc06..d838ba4c234f9 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/DataFrameSetOperationsSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/DataFrameSetOperationsSuite.scala
@@ -1642,6 +1642,43 @@ class DataFrameSetOperationsSuite extends SharedSparkSession with AdaptiveSparkP
       }
     }
   }
+
+  test("SPARK-51262: exceptAll after dropDuplicates with subset should not throw") {
+    // Data where dropDuplicates(subset) produces deterministic results - to avoid test flakiness.
+    val df1 = spark.createDataFrame(Seq(
+      (1, "a", 100),
+      (2, "b", 200),
+      (3, "c", 300)
+    )).toDF("id", "name", "value")
+
+    val df2 = spark.createDataFrame(Seq(
+      (1, "a", 100)
+    )).toDF("id", "name", "value")
+
+    // dropDuplicates with subset - each (id, name) is already unique so output is deterministic
+    val deduped = df1.dropDuplicates("id", "name")
+
+    // exceptAll should work without INTERNAL_ERROR_ATTRIBUTE_NOT_FOUND
+    val result = deduped.exceptAll(df2)
+    assert(result.columns === Array("id", "name", "value"))
+    val rows = result.collect().sortBy(_.getInt(0))
+    assert(rows.length === 2)
+    assert(rows(0) === Row(2, "b", 200))
+    assert(rows(1) === Row(3, "c", 300))
+
+    // Also verify except (non-all) works and returns correct values
+    val result2 = deduped.except(df2)
+    val rows2 = result2.collect().sortBy(_.getInt(0))
+    assert(rows2.length === 2)
+    assert(rows2(0) === Row(2, "b", 200))
+    assert(rows2(1) === Row(3, "c", 300))
+
+    // intersectAll should also work and return the matching row
+    val result3 = deduped.intersectAll(df2)
+    val rows3 = result3.collect()
+    assert(rows3.length === 1)
+    assert(rows3.head === Row(1, "a", 100))
+  }
 }
 
 case class UnionClass1a(a: Int, b: Long, nested: UnionClass2)