[fix](agg) fix rule merge_aggregate (apache#59629)

Related PR: apache#31811

Problem Summary:
Before this pr:
The LogicalAggregate generated by MergeAggregate outputExpressions has
duplicated column .
This bug will not lead to result wrong, will output the gby key 2 times
in LogicalAggregate.
After this pr:
This pr fix this problem.

Author: feiniaofeiafei

fe/fe-core/src/main/java/org/apache/doris/nereids/rules/rewrite/MergeAggregate.java

@@ -100,6 +100,7 @@ private Plan mergeAggProjectAgg(LogicalAggregate<LogicalProject<LogicalAggregate
         Map<ExprId, Expression> innerAggExprIdToAggFunc = getInnerAggExprIdToAggFuncMap(innerAgg);
         // rewrite agg function. e.g. max(max)
         List<NamedExpression> replacedAggFunc = replacedOutputExpressions.stream()
+                .filter(e -> e.containsType(AggregateFunction.class))
                 .map(e -> rewriteAggregateFunction(e, innerAggExprIdToAggFunc))
                 .collect(Collectors.toList());
         // replace groupByKeys directly refer to the slot below the project

fe/fe-core/src/test/java/org/apache/doris/nereids/rules/rewrite/MergeAggregateTest.java

@@ -0,0 +1,124 @@
+// 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.doris.nereids.rules.rewrite;
+
+import org.apache.doris.nereids.trees.expressions.Alias;
+import org.apache.doris.nereids.trees.expressions.NamedExpression;
+import org.apache.doris.nereids.trees.expressions.Slot;
+import org.apache.doris.nereids.trees.expressions.SlotReference;
+import org.apache.doris.nereids.trees.expressions.functions.agg.Sum;
+import org.apache.doris.nereids.trees.plans.Plan;
+import org.apache.doris.nereids.trees.plans.logical.LogicalAggregate;
+import org.apache.doris.nereids.trees.plans.logical.LogicalEmptyRelation;
+import org.apache.doris.nereids.trees.plans.logical.LogicalProject;
+import org.apache.doris.nereids.types.IntegerType;
+
+import com.google.common.collect.ImmutableList;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.lang.reflect.Method;
+import java.util.List;
+
+/**
+ * Unit tests for {@link MergeAggregate}, specifically testing the fix for filtering
+ * aggregate functions in mergeAggProjectAgg method.
+ */
+public class MergeAggregateTest {
+
+    private MergeAggregate mergeAggregate;
+
+    @BeforeEach
+    public void setUp() {
+        mergeAggregate = new MergeAggregate();
+    }
+
+    @Test
+    public void testMergeAggProjectAggWithMixedExpressions() throws Exception {
+        // This test verifies the fix at line 103-104 where we filter expressions
+        // to only process those containing AggregateFunction.
+        // The bug was that non-aggregate expressions (like SlotReference) were
+        // being passed to rewriteAggregateFunction, which could cause errors.
+
+        // Create inner aggregate: group by a, output a, sum(b) as sumBAlias
+        SlotReference a = new SlotReference("a", IntegerType.INSTANCE);
+        SlotReference b = new SlotReference("b", IntegerType.INSTANCE);
+        Sum sumB = new Sum(b);
+        Alias sumBAlias = new Alias(sumB, "sumBAlias");
+
+        LogicalEmptyRelation emptyRelation = new LogicalEmptyRelation(
+                org.apache.doris.nereids.trees.expressions.StatementScopeIdGenerator.newRelationId(),
+                ImmutableList.of());
+
+        LogicalAggregate<Plan> innerAgg = new LogicalAggregate<>(
+                ImmutableList.of(a),
+                ImmutableList.of(a, sumBAlias),
+                emptyRelation);
+
+        // Create project: projects = [a as colA, sumBAlias]
+        SlotReference colA = new SlotReference(
+                sumBAlias.getExprId(), "colA", IntegerType.INSTANCE, true, ImmutableList.of());
+        // Create a slot reference for sumBAlias from inner aggregate output
+        Slot sumBSlot = sumBAlias.toSlot();
+        LogicalProject<LogicalAggregate<Plan>> project = new LogicalProject<>(
+                ImmutableList.of(colA, sumBSlot),
+                innerAgg);
+
+        // Create outer aggregate: group by colA, output colA, sum(sumBAlias)
+        Slot col2FromProject = project.getOutput().get(0);
+        Slot col1FromProject = project.getOutput().get(1);
+        Sum sumSum = new Sum(col1FromProject);
+        Alias sumSumAlias = new Alias(sumSum, "sumSum");
+
+        // Outer aggregate output contains:
+        // 1. colA (SlotReference - non-aggregate, should be filtered out)
+        // 2. max(sumBAlias) (AggregateFunction - should be processed)
+        // 3. sumBAlias (SlotReference - non-aggregate, should be filtered out)
+        List<NamedExpression> outerAggOutput = ImmutableList.of(
+                col2FromProject,
+                sumSumAlias
+        );
+
+        LogicalAggregate<LogicalProject<LogicalAggregate<Plan>>> outerAgg = new LogicalAggregate<>(
+                ImmutableList.of(col2FromProject),
+                outerAggOutput,
+                project);
+
+        // Use reflection to call the private method
+        Method method = mergeAggregate.getClass().getDeclaredMethod(
+                "mergeAggProjectAgg", LogicalAggregate.class);
+        method.setAccessible(true);
+
+        // This should not throw an exception
+        // Before the fix, non-aggregate expressions would be passed to rewriteAggregateFunction
+        // which could cause errors. After the fix, only expressions containing AggregateFunction
+        // are processed.
+        Plan result = (Plan) method.invoke(mergeAggregate, outerAgg);
+
+        Assertions.assertNotNull(result);
+        Assertions.assertTrue(result instanceof LogicalProject);
+
+        LogicalProject<Plan> resultProject = (LogicalProject<Plan>) result;
+        Assertions.assertNotNull(resultProject.child(0));
+        Assertions.assertTrue(resultProject.child(0) instanceof LogicalAggregate);
+
+        LogicalAggregate<Plan> aggregate = (LogicalAggregate<Plan>) resultProject.child(0);
+        Assertions.assertEquals(aggregate.getOutput().size(), 2);
+    }
+}