fix: ignore repeated sort keys in an ordered aggregate's ORDER BY

datafusion/physical-expr/src/aggregate.rs

@@ -61,7 +61,7 @@ use datafusion_functions_aggregate_common::accumulator::{
 };
 use datafusion_functions_aggregate_common::order::AggregateOrderSensitivity;
 use datafusion_physical_expr_common::physical_expr::PhysicalExpr;
-use datafusion_physical_expr_common::sort_expr::PhysicalSortExpr;
+use datafusion_physical_expr_common::sort_expr::{LexOrdering, PhysicalSortExpr};
 
 #[derive(Debug, Clone)]
 struct AggregateHumanDisplay {
@@ -265,12 +265,18 @@ impl AggregateExprBuilder {
         assert_or_internal_err!(!args.is_empty(), "args should not be empty");
 
         // An order-insensitive aggregate ignores its ORDER BY, so drop it here.
-        // Everything derived from `order_bys` below, such as the ordering fields
-        // in the aggregate's state, then agrees that there is no ordering.
+        // Otherwise drop sort keys that repeat an earlier one: a repeated key can
+        // never break a tie the earlier one left (`ORDER BY b ASC, b DESC` orders
+        // like `ORDER BY b ASC`), and accumulators build their ordering with
+        // `LexOrdering`, which drops such repeats. Everything derived from
+        // `order_bys` below, such as the ordering fields in the aggregate's
+        // state, then matches the sort keys the accumulators compare.
         let order_bys = if fun.order_sensitivity().is_insensitive() {
             vec![]
         } else {
-            order_bys
+            LexOrdering::new(order_bys)
+                .map(Vec::from)
+                .unwrap_or_default()
         };
 
         let ordering_types = order_bys

datafusion/sqllogictest/test_files/aggregate.slt

@@ -8106,6 +8106,41 @@ c,a,b,d d,b,a,c
 statement ok
 drop table t;
 
+# A sort key repeated in an ordered aggregate's ORDER BY is redundant, and the
+# query must behave as if the key were listed once (issue #25398). Each `k` is
+# unique so every expected value is determined by the ordering.
+statement ok
+CREATE TABLE dup_sort_keys (g INT, k INT, v INT) AS VALUES
+  (1, 2, 20), (1, 1, 10), (2, 4, 40), (2, 3, 30);
+
+query IIII
+SELECT g,
+  first_value(v ORDER BY k, k),
+  last_value(v ORDER BY k, k),
+  nth_value(v, 2 ORDER BY k + 0, k + 0)
+FROM dup_sort_keys GROUP BY g ORDER BY g;
+----
+1 10 20 20
+2 30 40 40
+
+query I?T
+SELECT g,
+  array_agg(v ORDER BY k, k),
+  string_agg(CAST(v AS VARCHAR), ',' ORDER BY k DESC, k ASC)
+FROM dup_sort_keys GROUP BY g ORDER BY g;
+----
+1 [10, 20] 20,10
+2 [30, 40] 40,30
+
+query II
+SELECT DISTINCT ON (g) g, v FROM dup_sort_keys ORDER BY g, g, k;
+----
+1 10
+2 30
+
+statement ok
+drop table dup_sort_keys;
+
 
 # Tests for aggregating with NaN values
 statement ok